Initializing referenceview "add" toolbar via toolbar controller

The "add" toolbar for adding a new snak to a reference makes use of the toolbar
controller now. A toolbar definition accepts custom event now - added some more
detailed documentation regarding toolbar definitions.

Change-Id: Ic3806552f0c1057bbaf32dbebc9df99975b19485
Merge "Initializing referenceview "add" toolbar via toolbar controller"