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.
I think Dolphin needs to retain support for switching between fullscreen and windowed.