Also make SetMinSize() after wxSizer::Fit() work correctly

This fixes a problem similar to that fixed in the previous commit but
for SetMinSize(), which was also ignored if done after calling
wxSizer::Fit() and before showing the window, as the explicitly set min
size was also overwritten by the pending min size computed from the
client size corresponding to the sizer fitting size.

The fix is similar too: just invalidate the pending minimum size if
SetMinSize() is called.

include/wx/gtk/toplevel.h

@@ -141,6 +141,8 @@ public:
 #ifdef __WXGTK3__
     void GTKUpdateClientSizeIfNecessary();
 
+    virtual void SetMinSize(const wxSize& minSize) wxOVERRIDE;
+
     virtual void WXSetInitialFittingClientSize(int flags) wxOVERRIDE;
 
 private: