Partial support for remembering last launched full screen mode.

[dalchen]

This patch only works for toggling full screen via the hotkey. For some reason, QT's isFullScreen is false if you go from not full screen to full screen via the green macOS window button.

Tested the following on macOS (hotkey means configured hotkey for full screen):

-- Working cases:

• launch, quit, launch. Result: not full screen.
• launch, hotkey, quit, launch. Result: full screen.
• launch, hotkey, hotkey, quit, launch. Result: not full screen.
• launch, resize, hotkey, quit, launch (Result: full screen), hotkey. Result: not full screen with previously resized window size.

-- Not working cases (mouse means mouse click on green macOS window button):

• launch, mouse, quit, launch. Incorrect result: not full screen.
• launch, hotkey, mouse, quit, launch. Incorrect result: full screen.

[dalchen]

First time contributor here! This PR is for #936 but doesn't fix it all the way as described in the commit message. I have also only tested this on macOS. Let me know if this is helpful or if any of you have any suggestions, thanks!