Completion: Complete automatically only when in sync

Completion: Complete automatically only when in sync

Whith the out-of-GUI completion one can have undesired
effects when invoking the completion, continue typing,
getting automatic content added to the editor
asynchronously, and then noticing that the lastly
input characters were "shifted" to a different
position.

Now we only do partial completions or implicit completions
(when there's a single match) if the proposal widget is
still in synch with the editor. In other words, if the
editor has not received input from the moment the completion
was triggered until it was actually displayed.

Change-Id: I31683bd596c9f75b9a48bb79dafb48eec8f12a3e
Reviewed-on: http://codereview.qt.nokia.com/44
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>