Revert "StateMachineScene::{currentItem -> currentElement}"

Let's try to consistently use 'item' in the scene scope for now

This reverts commit 16474deb3ea4374d9a8e622eaf0fea1379f0f4b8.

Conflicts:
	src/view/qml/uml/UmlTransition.qml

Change-Id: Ic115009b43241a3ca8dda9d1f35d9054de6cccb1