Add Util::descendantOf utility function

Change-Id: I46512affec0db8292344266ffa8e840331f1690d
Fix crash when deleting items in event handler

Don't delete QQuickItems directly in our instantiator, use QObject::deleteLater

Change-Id: I64087d6a148ec4f05997d4c32def948ec20602e2
Make it possible to filter subtree by double-click

For example double-clicking on a composite state will now just show this
state and its substates + outgoing edges

Change-Id: I98ff6580b3b4a528cb780da777bac212da1f4faf