Clear all keydown events that correspond to a keyup event
Make keyup event suppression more readable
Remove insert mode check for keyup handling

If we have handled a keydown then we should always handle its
corresponding keyup, regardless of what mode we're in. This prevents
us from passing the keyup event for `i` to the page when we enter insert
mode.
Only register events as handled once
Merge branch 'stop-memory-leak-for-keydown-repeats' of https://github.com/mrmr1993/vimium into mrmr1993-stop-memory-leak-for-keydown-repeats
Rework KeydownEvents as an abject.
Fix second memory leak in KeydownEvents.
Merge pull request #1313 from smblott-github/mrmr1993-stop-memory-leak-for-keydown-repeats

Mrmr1993 stop memory leak for keydown repeats