don't cache width & height for CairoWidget; provide option for all CairoWidgets to...

don't cache width & height for CairoWidget; provide option for all CairoWidgets to not draw their backgrounds; provide a static method to allow any (Container, probably) widget to provide background color to a CairoWidget without the requirement that it have its own window; remove existing cursor from clocks; clock text no longer has background attribute set, to correct a mistake in my understanding of pango attributes

git-svn-id: svn://localhost/ardour2/branches/3.0@10673 d708f5d6-7413-0410-9779-e7cbd77b26cf