GROOVY-6395: Complete navigable properties for Map and Node instances

E.g.

    groovy:000> x = ['aaaaa': 1, 'bbbbb':2]
    ===> [aaaaa:1, bbbbb:2]
    groovy:000> x.a <tab>

will now complete to x.aaaaa

Node support is intended for e.g. XmlSlurper and ConfigSlurper
Merge pull request #308 from tkruse/groovy6395

GROOVY-6395: Complete navigable properties for Map and Node instances