Implement file search

Clear found files list on clear button pressed

Make found files header bold

Enable clearing of search box with button click

Add tool tip to search box

Filter data sets on pressing Enter

Add function to filter directories

Closes #10 (closed)

sohstationviewer/view/ui/main_ui.py

 # UI and connectSignals for main_window
-from typing import Union, List
+from typing import Union, List, Optional, Iterable
 
 from PySide2 import QtCore, QtGui, QtWidgets
+from PySide2.QtCore import QObject
+from PySide2.QtGui import QIcon, QPixmap
 from PySide2.QtWidgets import (
     QMainWindow, QWidget, QTextBrowser, QPushButton, QLineEdit, QDateEdit,
     QListWidget, QCheckBox, QRadioButton, QMenu, QAction, QLabel, QFrame,
     QVBoxLayout, QHBoxLayout, QGridLayout, QAbstractItemView, QShortcut,
-    QActionGroup, QButtonGroup
+    QActionGroup, QButtonGroup, QStyle,
 )
 
 from sohstationviewer.view.calendar.calendar_widget import CalendarWidget
@@ -90,9 +92,9 @@ class UIMainWindow(object):
         """
         self.search_line_edit: Union[QLineEdit, None] = None
         """
-        clear_button: clear search_line_edit
+        clear_button: clear search_line_edit 
         """
-        self.clear_button: Union[QPushButton, None] = None
+        self.clear_search_action: Optional[QAction] = None
         """
         search_button: filter list of files based on content of
             self.search_line_edit
@@ -391,41 +393,52 @@ class UIMainWindow(object):
             QAbstractItemView.ExtendedSelection)
 
         search_layout = QHBoxLayout()
-        # search_layout.setSpacing(6)
         left_layout.addLayout(search_layout)
         self.search_line_edit = QLineEdit(self.central_widget)
         self.search_line_edit.setPlaceholderText('Search...')
         self.search_line_edit.setToolTip('Filter the list of files based on '
                                          'the content.')
+        self.search_line_edit.textChanged.connect(self.main_window.filter_folder_list)
+        self.search_line_edit.setClearButtonEnabled(True)
+        try:
+            # This value was obtained from the C++ source of QT. We use it here
+            # so that the QAction found is guaranteed to be the clear button.
+            clear_action_name = '_q_qlineeditclearaction'
+            found_actions = self.search_line_edit.findChildren(
+                QAction, clear_action_name
+            )
+            self.clear_search_action: QAction = found_actions[0]
+            self.clear_search_action.triggered.connect(
+                self.main_window.clear_file_search
+            )
+        except IndexError:
+            # If the name of the clear button is changed in the C++ source of
+            # QT, nothing will be found. So, we raise an error to indicate this
+            # problem.
+            raise ValueError('No clear button could be found. Check its '
+                             'objectName attribute using QObject.findChildren '
+                             'without a name.')
         search_layout.addWidget(self.search_line_edit)
-        search_layout.addSpacing(2)
-        self.clear_button = QPushButton('Clear', self.central_widget)
-        self.clear_button.setFixedWidth(70)
-        search_layout.addWidget(self.clear_button)
-        search_layout.addSpacing(2)
-        self.search_button = QPushButton('Search', self.central_widget)
-        self.search_button.setFixedWidth(70)
-        search_layout.addWidget(self.search_button)
-
-        search_grid = QGridLayout()
-        left_layout.addLayout(search_grid)
+
+        read_option_grid = QGridLayout()
+        left_layout.addLayout(read_option_grid)
         self.from_data_card_check_box = QCheckBox(
             'From Data Card', self.central_widget)
-        search_grid.addWidget(self.from_data_card_check_box, 0, 0, 1, 2)
+        read_option_grid.addWidget(self.from_data_card_check_box, 0, 0, 1, 2)
 
         self.replot_button = QPushButton('RePlot', self.central_widget)
         self.replot_button.setFixedWidth(95)
-        search_grid.addWidget(self.replot_button, 0, 3, 1, 1)
+        read_option_grid.addWidget(self.replot_button, 0, 3, 1, 1)
 
         data_group = QButtonGroup()
         self.data_radio_button = QRadioButton('data', self.central_widget)
         data_group.addButton(self.data_radio_button)
         self.data_radio_button.setEnabled(False)
-        search_grid.addWidget(self.data_radio_button, 1, 0, 1, 1)
+        read_option_grid.addWidget(self.data_radio_button, 1, 0, 1, 1)
         self.sdata_radio_button = QRadioButton('sdata', self.central_widget)
         self.sdata_radio_button.setEnabled(False)
         data_group.addButton(self.sdata_radio_button)
-        search_grid.addWidget(self.sdata_radio_button, 1, 1, 1, 1)
+        read_option_grid.addWidget(self.sdata_radio_button, 1, 1, 1, 1)
 
         color_tip_fmt = ('Set the background color of the plot '
                          ' to {0}')
@@ -769,10 +782,6 @@ class UIMainWindow(object):
             main_window.filter_folder_list
         )
 
-        self.clear_button.pressed.connect(main_window.clear_file_search)
-
-        self.search_button.pressed.connect(main_window.filter_folder_list)
-
         self.replot_button.clicked.connect(main_window.replot_loaded_data)
 
         self.background_black_radio_button.toggled.connect(