implement a preferred order for POM elements

maven.mdo defines a de-facto order for POM XML elements, that is visible in POM reference documentation or XML documents generated with MavenXpp3Writer.

The Maven team has voted during the end of June 2008 to follow a specific POM convention to ordering POM elements to improve their readingness. The consequence of this vote is that the Maven project descriptor is no more considered as the reference for the ordering.

With MODELLO-224 fixed in Modello 1.2, Maven's internal order can be configured in maven.mdo to match the POM convention: doing so would improve POM reference documentation (the convention is more intuitive than the actual de-facto order) and make MavenXpp3Writer implement the convention.

Note that this is only a preferred but not enforced order: POM elements order remains unchecked.