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.
We could rename this setting to "bExclusiveFullscreen" or "bExclusiveMode" since it well now only be activated in case exclusive mode needs to be activated.