Treating modifier keys in a Macro combination as ghostkey

[wpc009]

A GhostKey is defined as a key event that is triggered in a macro combination. The GhostKey should not influence other key event that not included in this macro combination. Which means the GhostKey only affects the keys in same macro combination.

Since the current kll spec does not have a `GhostKey` concept. 
So, this impl will preventing mapping multiple modifier event on single key strike.