#36 Standardize and improve Ok/Cancel/Default buttons

[Alex Abdugafarov]

See https://github.com/frozenspider/tuxguitar/pull/4

All of this is mostly done so that you won't accidentally screw your settings.

• Panel for OK/Cancel/Default buttons has been standardized across all dialogs
• Default button is now left-aligned and asks for confirmation
• New control integrated into Fretboard, Piano and Matrix configs and Settings seamlessly
• New control also integrated into Key Bindings (Shortcuts) config, slightly changing the behaviour.
  • Changes are now only applied if OK button is pressed and not on any dialog closing (same as with other dialogs).
  • Changes are discarded if Cancel is pressed or dialog is closed (same as with other dialogs).
  • Default button resets the hotkeys, but doesn't close window nor does it apply changes immediately (unlike other dialogs). Pressing Ok/Cancel will interact with those changes as expected.

How it looks like: