trashmonitor: change trash monitoring process

trashmonitor: change trash monitoring process

The trash is monitored for state changes - going from empty to non-empty and the
other way round. Monitoring is done by handling change signals from a regular
file monitor. On each signal, an enumeration of the trash contents is started in
order to see if it is empty or not. This causes issues when many files are
trashed, because the gvfs trash backend is flooded with enumeration requests,
resulting in CPU usage spikes. In order to fix this, the "item-count" attribute
of the trash should be queried instead.

Replace asynchronous enumeration with asynchronous information query and update
the trash state based on the "item-count" attribute. Emit state change signal
only when the state actually changes.

https://bugzilla.gnome.org/show_bug.cgi?id=763600