implement speaker widget referencing for message bubbles

[Simon Poirier]

The current implementation of MessageBubble action and TextBubble widget use hardcoded positions for tail and the widget itself. It would be desirable to abstract this positionning by specifying the "speaking" widget instead of coordinates. The action would then, lookup the speaker fqdn using gtktools and fetch it's absoluto postition to position itself relative to it and with the tail pointing to it.
This does not need to be a perfect implementation, but it would make a decent default for the tutorial editor for actions to come.
Sample functional test could be:
position textbubble relative to button where
- bubble is entirely in the window
- bubble does not overlap widget
- bubble points towards widget