extracted and restructured undo manager;
added separate undo manager for context editor and lattice view;
see #33 and #66
Disabled positioning checks for node movement, see #125 and #140
Resolve #66 by adding undo/redo support.
Cleaned up undo manager for context editor. Logically this should work
now but the table is one bunch of crap…so it does not react to context
changes. Because there are that much issues open regarding the context
editor and my decision of a redesign I will tackle it all together in
time. See #33, #42, #43, #116