DolphinQt: remove non-render-to-main and add fullscreen.

Not sure what needs to be done here for exclusive fullscreen; on X11/Wayland I already have `BypassWindowManager` enabled which should provide some performance improvement.

There's also a new crash-on-exit but I think that's unrelated to this change (it looks like something is killing timers from another thread). I'll have to poke around and see what that is.
Switching in and out of fullscreen during gameplay is a very common use case, it's the de facto standard for any emulator.