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