Add missing keyboard shortcuts

[davidovski]

This PR aims to improve ease of usability throughout the DAW, adding useful keyboard shortcuts.

• Ctrl+D will discard changes when prompted to save changes (borrowed from GIMP)
• Ctrl + B/Ctrl + Shift + B to add/remove bars in song editor
• Shift+Q to quantize notes in piano roll

I felt like these actions were missing shortcuts and I have personally found these very helpful. I have added relevant tooltips where appropriate, however I'm not sure the best way to signpost the Ctrl+D shortcut. As this is my first pull request, any criticism is greatly appreciated. Thank you!

[Spekular]

• Ctrl + B/Ctrl + Shift + B to add/remove bars in song editor

FYI Shift + Insert and Shift + Delete already do this.

[davidovski]

Thanks for pointing that out, I've removed those two extra shortcuts and updated tooltips to reflect the existing shortcuts.

[JohannesLorenz]

@davidovski If you want to get this merged, it would be nice if you could fix PhysSong's comment and solve the merge conflict.