Feature #13690 (New): Allow plugins to be able to add relationship types

This is somewhat related to #7849 but (I think) involves significantly less work.

Firstly - can we make the TYPES constant a class variable (eg. @types)?
Secondly - can the ".freeze" on the object inside the TYPES constant (now a class variable called @types) be removed? (If necessary it could always be lazily frozen at some time later after the plugins have had a chance to add a relation...)

around source:trunk/app/models/issue_relation.rb@11711#L65

Unless I'm mistaken this should be all that would be required to give redmine plugins the authority to add/remove relationship types.

1. Eg, a plugin would be able to do something like this
   IssueRelation.class_eval("@@types[:supercedes] = 'Adding a relation'")