skins: make custom scintilla scrollbars opt-in
http://bugs.activestate.com/show_bug.cgi?id=99857

(integrated from komodo (trunk) change 80944 by marky)

git-svn-id: https://svn.openkomodo.com/repos/openkomodo/trunk@13263 b87d16e7-d29f-4ee1-b5ec-817d645d335f