If dataset have more than one keys, allow user to select another key to plot

closes #93 (closed)

• Create button "Plot Different Key" which is disabled by default.
• After data is loaded, if there are more than one key for the data, the button will be enabled.
• User click on that to access a dialog that allow user to select a different key. Suppose that all data have been finalized right after loaded, we don't have to do finalized here but have to wait until !142 (merged) is merged
• Then replot_loaded_data() will be call to replot the selected data.

**Should retest this for new RT130 **