Adding support for types who have a static valueOf(String) method

[tom]

if a type set has a valueOf() method using a simply string in fxgraph should be proposed

See ​https://groups.google.com/forum/?fromgroups#!topic/efxclipse/kt0gPwHZNfI

[tom]

This is important for Color definitions as well. So we should maybe provide an extension mechanism which can implement proposals for different types and Validations of the values