(bug 44665) start/stopEditing events for claimview widget

This change set implements startEditing and stopEditing events in the claimview widget which
required some refactoring in claimlistview/listview widgets.
Since there is no point in highlighting the main snak's property when adding/editing a reference,
this change set removes the highlighting as well.

- patch set 3: Fixed behavior when canceling and reverted changes of patch set 2 since
these were not implemented correctly.

Change-Id: Ic6d28b436c231fa9a5e08aa0a00619b84d3deef5