Improve keyboard consistency

[RoganDawes]

Occasionally, the keystroke pipeline may stall, leaving a keydown event as the last thing sent to the target. If the stall is long enough, this can result in keyboard repeat activating on the target OS, ruining whatever was being typed.

The HID code (or common code) could introspect the HID events being issued, and ensure that key down events are only sent when the corresponding key up event is already in the pipeline.