The primary reason for the move is to be able to disable mouse keys
completely, and not even compile them in. For this to work, it needs to
be in a separate library, otherwise it will always be included, even if
not active.
So, this patch turns mouse-keys into a simple plugin, included with the
core firmware! This makes the default event handler a bit simpler, the
code marginally smaller, and the feature completely optional.
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>