fix wxWindow::PushEventHandler and related wxWindow functions for the stack managemen...

fix wxWindow::PushEventHandler and related wxWindow functions for the stack management; currently they don't work well when passing event handlers which are part of an event handler chain (see wx-dev thread 'wxWindow event handler stack'); implement wxEvtHandler Unlink() and IsUnlinked() functions and document them; revise docs of all involved functions of both wxEvtHandler and wxWindow, adding images for better explanations

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58291 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775