Support for buttons inside rhaplets

In some cases it may not be enough for a rhaplet to display text, icon and a progressbar. Some times it may be needed to have a button that the user can press.

In order to achieve this we need to:
* Change the API between Dynapt and content engines to support for the engine placing buttons inside items and providing a callback function for it.
* Adapt Rhamphor for the new API and also change the API between Rhamphor and rhaplets, so the rhaplet can add buttons and supply callback functions for them.
* Change Tricera to paint buttons and connect them to the callback functions.