Fail instead of crashing in wxGTK wxCursor::CloneGDIRefData().

Fail instead of crashing in wxGTK wxCursor::CloneGDIRefData().

CloneGDIRefData() used wxCursorRefData copy ctor which wasn't implemented
correctly and simply copied its internal m_cursor member without incrementing
its reference count which resulted in a crash when it was then dereferenced
twice.

Unfortunately there doesn't seem to be any simple way to clone GDK cursors but
as this should be something only rarely (if ever?) needed, simply don't
implement CloneGDIRefData() at all for now and just leave an assert in it if
it's ever really called. Also don't define wxCursorRefData copy ctor at all as
it can't be done correctly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66371 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775