backend: Update cursor visibility based on device availability

On startup, the cursor is kept hidden if there's any touchscreen available.
If the device that was last interacted is removed, we check on available
pointing devices though, so we don't possibly hide the pointer if there are
further mice/touchpads/etc.

Devices being added don't update cursor visibility, we wait for the user
interacting with those instead.

https://bugzilla.gnome.org/show_bug.cgi?id=712775