Introduce wxAnyButton class, a common base class for wxButton and
wxToggleButton, allowing to reuse the same implementation for them.
This also allows to implement support for bitmaps in wxToggleButton for all
platforms and make wxBitmapToggleButton a trivial subclass of it everywhere,
similarly to wxBitmapButton and wxButton.
Closes#13198.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67931 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
