UI: Add simple support for a custom background.

No UI.  Essentially, this is a cleaned up version of xiushudongfang's
implementation that supports all platforms, but no selection UI.

UI/MainScreen.cpp

@@ -985,8 +985,6 @@ UI::EventReturn MainScreen::OnLoadFile(UI::EventParams &e) {
 	return UI::EVENT_DONE;
 }
 
-extern void DrawBackground(UIContext &dc, float alpha);
-
 void MainScreen::DrawBackground(UIContext &dc) {
 	UIScreenWithBackground::DrawBackground(dc);
 	if (highlightedGamePath_.empty() && prevHighlightedGamePath_.empty()) {