Initial wxTLW::(Can)SetTransparent support for wxGTK. The return value of SetTranspar...

Initial wxTLW::(Can)SetTransparent support for wxGTK. The return value of SetTransparent is currently very
optimistic and doesn't use the EWMH composite manager spec update proposal (_NET_WM_CM_Sn) yet. This is on purpose
until the proposal appears in EWMH new version draft and it gets used by more composite managers.
See http://www.nabble.com/Interaction-between-applications-and-compositing-managers-t1389248.html and in the
future http://standards.freedesktop.org/wm-spec/latest/ for details.
Some code reordering is probably in order (helper function to utilsx11?) and testing on non-traditional X servers
appreciated.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@40192 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775