Forward events to active child in wxMDIParentFrameBase, not only wxMSW.

Menu, toolbar and update UI events should be forwarded to the active child
under all platforms in MDI applications and not only MSW, so move TryBefore()
overload from wxMSW wxMDIParentFrame to wxMDIParentFrameBase.

This should make things more consistent in general and notably fixes the lack
of toolbar events in wxDocChildFrame since the changes of r73928.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74314 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/mdi.h

@@ -110,9 +110,6 @@ public:
 #endif // wxUSE_MENUS
 
 protected:
-    // override to pass menu/toolbar events to the active child first
-    virtual bool TryBefore(wxEvent& event);
-
 #if wxUSE_MENUS_NATIVE
     virtual void InternalSetMenuBar();
 #endif // wxUSE_MENUS_NATIVE