VideoRenderers: Use a focus window in D3DFullscreen.

Also, don't allow D3D to change the window style. This fixes the loss of focus when D3DFullscreen is active.
FullscreenWnd: Send thumb mouse buttons to MainFrm.
MainFrm: Exit D3DFullscreen at the end of playback if enabled in the options.
MainFrm: Don't open the popup menu behind D3DFullscreen.
MainFrm: Only start in D3DFullscreen if we closed it, remembered it or indicated it on the command-line.
FullscreenWnd: Exit fullscreen if focus is switched to a window behind the D3DFS window.