Factor out icon loading to Screen::LoadIcon()

This is so it can be called multiple times without having to duplicate
code.

desktop_version/src/Screen.h

@@ -16,6 +16,8 @@ public:
 		bool badSignal
 	);
 
+	void LoadIcon();
+
 	void ResizeScreen(int x, int y);
 	void ResizeToNearestMultiple();
 	void GetWindowSize(int* x, int* y);