Exclusive fullscreen fixes

Fixes [ticket 7516](https://code.google.com/p/dolphin-emu/issues/detail?id=7516) and a few other bugs.

Bugs fixed:
* "Use Fullscreen" ignoring the borderless fullscreen setting.
* Borderless fullscreen setting being reset by the video config dialog for backends that don't support it.
* Window size being forgotten when exiting the game while in borderless fullscreen.