VideoConfig: Add exclusive mode flag.

Allows the UI to easily check the current exclusive mode state.
This simplifies a few checks and prevents the user from ever getting stuck in fullscreen.

Source/Core/VideoCommon/VideoConfig.cpp

@@ -28,7 +28,10 @@ void UpdateActiveConfig()
 VideoConfig::VideoConfig()
 {
 	bRunning = false;
+
+	// Exclusive fullscreen flags
 	bFullscreen = false;
+	bExclusiveMode = false;
 
 	// Needed for the first frame, I think
 	fAspectRatioHackW = 1;