comment Ctrl-V in Firefox/Opera test case #comment #doc

comment test case with what actual situation is expected to produce that
sequence of events
test for Ctrl-V in Firefox/Opera with input event

because new versions of Firefox/Opera also fire an `input` event in
between the `paste` event and the timeouts expiring
fix #149: clear stale timeouts in textarea manager

whenever we set a timeout, the previous timeout is now obviously stale,
so clear it if possible (clearTimeout() fails silently)