[wxGTK] Be pessimistic about transparency being visually shown in CanSetTransparent...

[wxGTK] Be pessimistic about transparency being visually shown in CanSetTransparent. With current code
CanSetTransparent will only return TRUE if gtk+ version is at least 2.10 AND there is a running compositing
manager that implements the _NET_WM_CM_Sn EWMH spec addition proposal (at this point only metacity GL based
compositor, afaik). This is for wxAUI to be able to implement dock preview correctly. In general these behaviours
needs some discussion.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@40193 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775