backend: Add meta_backend_update_last_device()

This function can be used to trigger changes depending on the device type
that is currently emitting the events. So far, it is used to switch cursor
visibility on/off on touchscreen interaction.

A "last-device-updated" signal has also been added, in order allow hooking
other behavior changes (eg. OSK) when the last device changes.

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