add support for Java8 type annotations

[tmtron]

xbase should support Java8 type annotations, so that we can generate code like this in the JvmModelInferrer:

public final @IdInstance long id;

• Reference to Stackoverflow: how to use type annotations in xtext?
• maybe related xtend issue 443305 - Support type annotations

[cdietrich]

i wonder if this can easily done. at least in xtend this will lead to ambiguities with fields with no var/val keyword