improved checkEdges to stop, if there is no single possible reference
Merge branch 'uni-lu-dev' of
git@github.com:de-tu-berlin-tfs/Henshin-Editor.git into uni-lu-dev

Conflicts:
	de.tub.tfs.henshin.tgg.editor/src/de/tub/tfs/henshin/tggeditor/commands/FTRuleConstraintE.java