Allow to define transient attributes in templates (#87)

[StephanErb]

To allow for the use of mutiple tokens within a query that are recognized separately something like "transient attributes" in templates would be useful.

For example:
template A
: ${a} = "#" template{as=identifier,
query=Template.allInstances()->select(name=${a} and mode=?)}
;

"a" is not a property of class A but is required in the query. The above example syntax could be used to define an extra token "a" in the template for which the parse rule "identifier" is called. The value of "a" could then be used in the query to resolve a model element for "template".