Fractional scaling support

Use resource scale for st-widgets, and other (fractional) scaling fixes.

There are still some commits that are in RFC or needs to be squashed, but I wanted a feedback first.

Depends on mutter!3 (merged).

There are still some TODO's: https://hackmd.io/WspOFZpRTo2qlWc8fh_zPQ?view#TODO

As per https://bugzilla.gnome.org/show_bug.cgi?id=765011