Prior to this commit, setting up a zero timeout resulted in a timer that never
expires, which isn't particularly useful.
This commit allows setting up timers that expire immediately, which is useful
to immediately enter idle state.
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.
