clutter: Make font rendering optional

See the various commits. It allows building Mutter without any fonts rendering dependencies: pango / harfbuzz / fribddi. Once we have migrated the remaining meta APIs away from cairo, we can make cairo specific to fonts rendering / x11 only.

This adds a build time option, -Dfonts=false|true, that is exported in the pkg-config, so compositors like shell could bail out if mutter is built without text rendering support.

Marked as a draft as this is a proposal, not all the commits would make sense as is and might require some further discussions but we can cherry-pick some of them already.

Long term, we would make those build time options runtime modules similar to how weston currently do, but for now this should allow us to provide a good base for using Mutter as a compositor in embedded use cases

Requires gnome-shell!3544 (merged)