make wxControlContainer accept focus depending on whether it has any focusable children when using native TAB navigation too but also allow to manually override this automatic detection; added wxWindow::SetCanFocus() to notify GTK+ about changed focus state

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45267 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/window.h

@@ -68,6 +68,7 @@ public:
     virtual bool IsRetained() const;
 
     virtual void SetFocus();
+    virtual void SetCanFocus(bool canFocus);
 
     virtual bool Reparent( wxWindowBase *newParent );