cursor: Show the cursor only when a mouse gets used

- On startup, we're responsible for showing the cursor when
the mouse gets moved the first time.
- During run-time, when switching to a touchscreen, hide the cursor,
  and show it again when switching back to the mouse.

(Note: in the above text, mouse is a shortcut to "pointer device
 that isn't a touchscreen, it works just as well for touchpads)

https://bugzilla.gnome.org/show_bug.cgi?id=687791