Added missing text box to main UI; fixed erroneously labelled buttons in toolbar; linked calendar buttons in toolbar to calendar dialog

Moved ui.py files into view/ui/; added absolute imports to ui implementations and added sohstationviewer.py as entry point.

Changed signal used by calendardialog implementation to enable keyboard input for date selection in addition to mouse input.

Set initial path for widget displaying selected data directory to the current working directory at startup

Init structure

See merge request passoft/sohstationviewer!1