keyboard: cancel repeat when handling key-bind

keyboard: cancel repeat when handling key-bind

<keybind key="W-d">
  <action name="Execute">
    <command>dmenu_run</command>
  </action>
</keybind>

When using the keybind above (in rc.xml), on the first execution of W-d
all is okay, but the second time, a "d" pressed event is sent to dmenu
resulting in a continuous "ddddddd...") which has to be stopped pressing a
key.

This behaviour started in commit 7e57b7f because release events associated
with keybinds are no longer sent to clients (before that commit, the
release event for the “d” would have been passed to dmenu, thus cancelling
the repeat).

Solves issue #176

Helped-by: @spectrum70