There was no way to tell wlr_idle to stop processing input events
and rearm timers all the time, such an API is required to have
some form of idle inhibitor.
This moves the `struct wl_signal activity_notify` in `struct wlr_idle`
into a local `struct {} events` to keep consistency with other modules
in the library.
Dominique Martinet
Markus Ongyerth
Drew DeVault
Scott Anderson
emersion
Heghedus Razvan