The terminal keydown handler was set to filter out all key bindings that
have a registered global handler, regardless of whether they actually
propagated or not. This allowed the global handlers to still work
despite the terminal input having precedence, but it also meant that
global key bindings that were invalid for the current context would
still get eaten and not sent to stdin.
Now, the terminal keydown handler will directly call the global handlers
so we can actually see whether or not the global key binding is valid.
If the global handler is valid, it'll be processed immediately and stdin
won't receive the input. If it's not handled, we'll let xterm pass it to
stdin. Because anything xterm doesn't handle gets sent to the
globally-registered version of the handler, we need to make sure we
don't do extra work to process an input we've already checked. We'll
store the last-handled keydown event as a static variable so we can
dedupe later calls for the same event to prevent doing double work.
