Fix bug with showing TLW in wxMSW when restoring its geometry

Since the changes of 6ae7aa4443, the
windows were shown when their geometry was restored as a side effect of
calling ::SetWindowPlacement(). This was unexpected and resulted in
flicker on startup, so fix this by explicitly passing SW_HIDE to
SetWindowPlacement() if the window is currently hidden and storing the
real show command inside wxTLW itself, where it will be used when it's
finally shown.

include/wx/msw/toplevel.h

@@ -116,6 +116,9 @@ public:
     // returns true if the platform should explicitly apply a theme border
     virtual bool CanApplyThemeBorder() const wxOVERRIDE { return false; }
 
+    // This function is only for internal use.
+    void MSWSetShowCommand(WXUINT showCmd) { m_showCmd = showCmd; }
+
 protected:
     // common part of all ctors
     void Init();