Reset modifier only hotkey activation between key presses

If a different modifier was pressed between two 'activation modifier' key presses, QS could still be activated.
E.g. in a 'double shift activation' scenario, if ⇪⌥⇧ was pressed, QS would still activate.
This change ensures that the state of any modifier events is reset when any non-related keys are pressed.

Fixes #1950
Merge pull request #1969 from quicksilver/doubleKeyActivation

Reset modifier only hotkey activation between key presses