Make ContextPattern pluginMethods more useful, flexible and effective

The syntax provided for specifying plug-in methods to extend ContextPatterns makes them hard to use. When specified, they replace both the "relation" and "token_name" fields in the context pattern, for example:

*,{swm}Swimlane,{cts]pluginMethod:getSwimlaneContents

The problem with this is that it is not possible to filter the return from the plugin method. If we wish to (in the example shown) separately access swimlane elements which are CallOperations, CallBehaviors and Actions, we need to write THREE plugin methods:

*,{swm}Swimlane,{cts]pluginMethod:getSwimlaneCallOperations
*,{swm}Swimlane,{cts]pluginMethod:getSwimlaneCallBehaviours
*,{swm}Swimlane,{cts]pluginMethod:getSwimlaneActions

This gets very unmanageable very quickly. A better approach would be to treat a plugin method as though it is a "relationship" rather than a "relationship:token_name" pair. In this case, we would be able to flexibly access different types of element with a single plugin method:

*,{swm}Swimlane,{cts]pluginMethod(getSwimlaneContents):CallOperation
*,{swm}Swimlane,{cts]pluginMethod(getSwimlaneContents):CallBehavior
*,{swm}Swimlane,{cts]pluginMethod(getSwimlaneContents):State

NB: The request does not specifically relate to getting the contents of swimlanes, which is used only as an example. This request is for a general improvement to plugin methods which will mean we can provide many fewer, much more useful extensions.