build: disable libnm-glib support by default

By now nobody should be using this. Keep the code around for a little
longer just in case anybody still uses this.

The libnm-glib support also serves as an example how do we build two
different versions of the properties plugin. We'll soon be in a similar
situation with Gtk 4.0. (sigh.) Just don't drop it yet.
2 jobs for lr/libnm-glib-disable in 5 minutes and 49 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #127473
fedora28_dist

00:03:19

 
  Test
passed #127474
fedora_from_dist

00:02:29