1. generate the events from the event handlers, thus it is now possible to
process the splitter events in the parent window
2. when double clicking the splitter which can't be unsplit it doesn't jump
by a couple of pixels
3. misc code cleanup
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@14263 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775