Implement idle-inhibit protocol

Some apps, like mpv, use idle-inhibit protocol in order to signalize that the screen is in use and shouldn't get blanked/locked (like, when watching a video).

So far we claim to support it as we create an object in src/desktop.c:

self->idle_inhibit = wlr_idle_inhibit_v1_create(server->wl_display);

...but we never do anything with it again.

It seems like what's needed is listening for idle_inhibit events and calling wlr_idle_set_enabled on self->idle when necessary.