add support for libmutter-14

[decathorpe]

With mutter 46.alpha, the library name changed again, to libmutter-14.

I can't tell if there were many breaking API changes, but apparently "there were somewhat intrusive changes on the clutter side, like killing off ClutterContainer" to quote what fmuellner wrote in the Fedora Workstation Matrix channel.

mutter 46.alpha is available in Fedora Rawhide for testing.

[danirabbit]

This was fixed in 5d22d43