Fix type inferencing, syntax highlighting and code navigation for extension method that overloads declared method(s)

[eric-milles]

Follow up from #1076. The situation extends beyond Object methods. Consider the following:

def map = new HashMap()
map.equals([:]) // DefaultGroovyMethods#equals(Map,Map)
map.toString() // DefaultGroovyMethods#toString(AbstractMap)
println(map) // Script#println(Object) -- not DefaultGroovyMethods#println(Object)

The runtime selection for "equals", "toString" and "println" are indicated in the comments. Highlighting for "equals" and "toString" should indicate an extension method (purple not green).

[eric-milles]