add support for input location tracking in Xpp3 DOM

[hboutemy]

working on input location tracking for Maven model and its display in help:effective-pom (see https://issues.apache.org/jira/browse/MPH-160)

plugins configuration are stored as Xpp3Dom objects read by Xpp3DomBuilder: we need input location tracking at that level

we need also to keep input location while merging with Xpp3Dom.mergeXpp3Dom(...)

[hboutemy]

#62 merged to master