Allow Element Creations only when they would be valid

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Add Create Constraints:
- link variable only between adequate variable (types) e.g.
- only edge guards that are valid between the context nodes

- this could be done via emf validation framework. to be discussed.

Original issue reported on code.google.com by aljoschability on 3 Apr 2012 at 9:49

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this issue could be devided into two (or more) "sub issues": create tools, 
property page, ...

on the property page some of the contraints are already integrated:
- on a link you can only select those target ends, that are valid between two 
objects (with an eclassifier)
- ObjectVariable, LinkVariable: only valid states of the bound state, operator 
and semantics can be selected
- ActivityEdge: some constraints are implemented regarding the EdgeGuard 
(EACH_TIME, END only on outgoing edge from ForEachStoryNode, outgoing from 
StartNode cannot have a Guard...)

Original comment by aljoschability on 25 Apr 2012 at 7:34

• Changed state: Started