Only block keyup for keys where we have handled a keyup/press event

Fixes the issue of Vimium indiscriminately capturing keyup events, introduced by PR #978.
I'm not sure, but I expect not. I think we've been letting the functions using `handlerStack` do their own event suppressing, so that they can eg. let certain keys pass through to the page.

I'll try and have a look later to see if this is a concern, but we don't seem to do it anywhere else.