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'd rather not support that, as Qt window reparenting is messy and I'd like to avoid it if possible... How many users really want that feature?