Add support for returning item location to wxGTK wxMimeTypesManager.

This is a squash merge of gtk_mimetype branch from
https://github.com/Hanmac/wxWidgets.git with some extra minor cleanup.

Closes https://github.com/wxWidgets/wxWidgets/pull/293

src/gtk/app.cpp

@@ -31,6 +31,7 @@
 #include <gtk/gtk.h>
 #include "wx/gtk/private.h"
 
+#include "wx/gtk/mimetype.h"
 //-----------------------------------------------------------------------------
 // link GnomeVFS
 //-----------------------------------------------------------------------------
@@ -435,6 +436,10 @@ bool wxApp::Initialize(int& argc_, wxChar **argv_)
         return false;
     }
 
+#if wxUSE_MIMETYPE
+    wxMimeTypesManagerFactory::Set(new wxGTKMimeTypesManagerFactory());
+#endif
+
     // we cannot enter threads before gtk_init is done
     gdk_threads_enter();