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.
Ok, just tested it, all you need is to add the line `g_Config.bFullscreen = !g_Config.bBorderlessFullscreen;` here and then add `#include "VideoCommon/VideoConfig.h"` to the inclusions at the top.

That adds D3D exclusive fullscreen support. You already had OpenGL exclusive fullscreen support, though you probably don't support borderless fullscreen on it.