Intermediate commit for UndoManager
Introduced GraphChangeListener.
Merge branch 'undo' of github.com:jgralab/jgralab into undo

Conflicts:
	src/de/uni_koblenz/jgralab/Graph.java
	src/de/uni_koblenz/jgralab/impl/GraphBaseImpl.java
	src/de/uni_koblenz/jgralab/utilities/gui/undo/GraphUndoManager.java
	testit/de/uni_koblenz/jgralabtest/undo/UndoTest.java