libtracker-miner: Make FANotify monitor a subclass of GLib monitor

And fallback into GLib for the situations that FANotify cannot set up a mark despite being detected at runtime.

Unfortunately, a growingly common situation for this are btrfs subvolumes. These are currently problematic with FANotify due to filesystem ID mismatches, thus EXDEV is currently returned. See https://lore.kernel.org/all/CAOQ4uxjt2+BECu60aisZs_D6pgHK3VbBjju6hzmvG=Ls88UqUA@mail.gmail.com/ for a description of the problem.

Hopefully this situation will be resolved at some point, in the mean time, transparently fallback to the old situation for users of btrfs.

Closes: #212 (closed)