In short: make things happen on key release rather than on keypress.

This allows us to do some fancy stuff like:

• Let longpress act as modifier (like shift) instead of useless aaaaaaautorepeat
• Let the same key act both as character/action or modifier, depending on it's pressed alone or not
• arbitrary translation of "chords" into key sequences, (for simple abbreviations, not as fancy as i e plover)

Requires Python 3, Pygobject, Ibus and Gtk