This experimental feature can be controlled with a flag in the advanced
settings panel.

In eventHandler.py keep track of objects that have had focus (on the object, using
attribute 'wasGainFocusObj').
This expands on the 'lastQueuedFocusObject' concept.

A cancellable speech command is added to speech sequences that result
from the focus change event.
This command is able to check if the object once had focus
('wasGainFocusObj') and if it still has focus.
Speech for objects that no longer have focus can be discarded,
or cancelled if already with the synth.

Then checking for cancellations is done both early (eventHandler) and late
(speech.manager.speak)

This requires some careful tracking and processing in speech manager.
When nothing is with the synth, try pushing more.
Otherwise, there are items in the queue but nothing being spoken.
When cancelling synth clear tracking of "with synth"

Use _removeCompletedFromQueue rather than _handleIndex
_handleIndex may not actually call _removeCompleted. Cancelled speech does
need it's callbacks called.

For now CancellableSpeechCommand should be considered a private API