contextupdate-1003761: Allow checking for incomplete updates

The reason for this change is documented in the comments for isUpToDate.
This allows us to do less checking in getAssignmentAt().

Signed-off-by: Joe Walker <jwalker@mozilla.com>
contextupdate-1003761: Propagate context updates with new event

Requistion gains an onExternalUpdate event which we call when
context.update or context.updateExec are called. We wire this in to
call textChanged so the the UI updates itself properly.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
contextupdate-1003761: Don't assume HTML constructors

Because Gecko.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
Merge branch 'contextupdate-1003761'