Call wxEvtHandler::TryBefore() only once from ProcessEvent().

Call wxEvtHandler::TryBefore() only once from ProcessEvent().

The event pre-processing hooks associated with the window should be called
only once during the event processing, we don't need to call TryBefore() for
each and every event handler associated with the window too.

This makes the code slightly simpler and faster and shouldn't change the
behaviour of any existing code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64260 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775