Factor out wxTopLevelWindowGTK::GTKDoAfterShow()

Just extract the code generating wxEVT_SHOW for TLWs in wxGTK in its own
function before modifying it to avoid having to do it in two places.

No real changes, this is a pure refactoring.

include/wx/gtk/toplevel.h

@@ -136,6 +136,8 @@ public:
 
     void GTKUpdateDecorSize(const DecorSize& decorSize);
 
+    void GTKDoAfterShow();
+
 protected:
     // give hints to the Window Manager for how the size
     // of the TLW can be changed by dragging