DX9AllocatorPresenter: Use the MainWnd as the focus window in D3DFullscreen.

Also, don't allow D3D to change the window style. This fixes the loss of focus when D3DFullscreen is active.
Exit fullscreen if focus is changed to a window on the same monitor even with multi-monitor setups.

This commit reverts 7e22ddca36d0556f8ef30cc8cae1aab40b746cba.