Host: Add a new "UIHasFocus" hook to determine if the UI has focus

We can't use RendererHasFocus for this purpose because of some issues
with exclusive fullscreen, and the new RendererHasFocus implementation
didn't work for non-Render to Main Window cases, since the renderer
window wasn't managed by wx.

Source/Core/Core/Host.h

@@ -23,6 +23,7 @@
 // The host can be just a command line app that opens a window, or a full blown debugger
 // interface.
 
+bool Host_UIHasFocus();
 bool Host_RendererHasFocus();
 void Host_ConnectWiimote(int wm_idx, bool connect);
 void Host_GetRenderWindowSize(int& x, int& y, int& width, int& height);