keyboard: Implement key repeat for keybindings

keyboard: Implement key repeat for keybindings

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