Posting of the .uno:Something commands is asynchronous.  To be able to find
out when eg. .uno:Save finished, this commit introduces a callback that fires
when that happens.

To be able to receive such a notification, the appropriate postUnoCommand()
must be called with 'true' as the parameter for bNotifyWhenFinished (defaults
to 'false').

Change-Id: I254939ebc8ea5f309ae39686dcaaeddd5148b0c9