Add GUI tests

[Godley]

Testing coverage is currently quite low due to having 0 tests for any of the GUI files or the application file.
click here for a proposed way of testing pyqt

[Godley]

Test list:

main window

• test sidebar buttons add the right widgets and unload them on double click
• test sidebar buttons call the appropriate signals (on_WidgetSignal, onPlaylistsReady etc)
• test sidebar buttons call the appropriate signals with files existing in the folder and the widget loads appropriate sub widgets
• test actions produce import dialog for import, return to setup window for new collection
• test theme actions apply appropriate stylesheet

Scorebook widget

• test double clicking a piece loads it in the main window and closes scorebook widget
• test changing the sort mode changes the order of the list
• test double clicking a piece loads it in the main window, then test piece info widget loads the right info

Playlist widgets

• test double clicking a playlist opens playlist in main window
• test my playlist add button loads new playlist widget
• test creating a new playlist adds it to the list
• test double clicking a piece in a loaded playlist, then opening the playlist browser loads the appropriate playlist + highlights the current piece

Sub windows

Setup window

• test selecting a folder loads the main window
• test pressing cancel on folder browser returns to setup window and doesn't open main window
• test clicking "new collection" from main window shows previous collection in collection list
• test clicking a collection then clicking delete removes it from the list. Also test music.db file in that folder is deleted

Add playlist window

• test search provides suggestions
• test removing pieces
• test add puts the playlist into the db

Import window

• test adding a file
• test cancelling adding a file
• test adding a file which isn't an xml or pdf