Fix regression with MDI children accelerators

Since the changes of 8034e35391 (see #16870)
accelerators, including the standard ones such as Ctrl-F4 under MSW, didn't
work any longer inside the MDI children.

Fix this by extending IsTopNavigationDomain() to allow for checking whether
the given window should stop propagation of all keyboard events, as wxTLW
does, or only TAB navigation ones as wxMDIChildFrame and wxAuiFloatingFrame
do.

include/wx/toplevel.h

@@ -246,7 +246,7 @@ public:
     // override some base class virtuals
     virtual bool Destroy() wxOVERRIDE;
     virtual bool IsTopLevel() const wxOVERRIDE { return true; }
-    virtual bool IsTopNavigationDomain() const wxOVERRIDE { return true; }
+    virtual bool IsTopNavigationDomain(NavigationKind kind) const wxOVERRIDE;
     virtual bool IsVisible() const { return IsShown(); }
 
     // event handlers