It seems that every Wayland client is expected to implement its own
key-repeat logic, rather than doing it server-side as in X11. This
means that labwc also has to implement its own key-repeat logic for
compositor keybindings.
This is a very simplistic timer-based implementation. It doesn't
attempt to synthesize accurate timestamps, and may lag depending
on system load, but it appears to get the job done.
v2: Use server->wl_event_loop
v3: Comments and formatting