Store "bgmain" in the GMenu2X object instead of in SurfaceCollection

Since this surface is created by initBG instead of loaded from skin
search paths, it didn't really fit in SurfaceCollection. After removing
it, one of SurfaceCollection's methods could be removed as well.

src/surfacecollection.h

@@ -44,12 +44,6 @@ public:
 
 	void debug();
 
-	/**
-	 * Adds a copy of the given surface to this collection under the given
-	 * path. Returns the copy.
-	 */
-	OffscreenSurface *add(Surface const& s, std::string const& path);
-
 	OffscreenSurface *add(const std::string &path);
 	OffscreenSurface *addSkinRes(const std::string &path, bool useDefault = true);
 	void     del(const std::string &path);