#3494 ColorObjectModel can tell if opacity can be set or not, and pass to the listener (now all GeoElements can set opacity but GeoButton. Fine tuning may be needed). Applied to desktop and web.

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@30682 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e