ENH: Improve display of disabled extensions

Change extension manager to append '(disabled)' after the extension name
when an extension is disabled, and to use disabled text color, which is
both more consistent with the icon display, and improves recognizability
of disabled extensions.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23168 3bd1e089-480b-0410-8dfb-8563597acbee