keyboard: break up handle_compositor_keybindings()

keyboard: break up handle_compositor_keybindings()

This function has grown quite large over time. Breaking out various
smaller functions makes the logic easier to follow.

No functional change intended, but there is a minor logical change:

- Due to factoring out match_keybinding(), each keypress can only
  match a single keybinding now. Previously, it was theoretically
  possible for a single keypress to map to multiple keysyms which could
  each match a different keybinding.