xtend-maven-plugin should not show optional warnings

Until there is a full-blown solution for [1], the xtend-maven-plugin should not show optional warnings, i.e. Xtend warnings that can be configured by the user inside the Eclipse IDE (via .settings/org.eclipse.xtend.core.Xtend.prefs) should not be shown by the maven plugin.

EXAMPLE: as of Xtend 2.16.0, the xtend-maven-plugin is showing warnings for unnecessary modifiers on fields/methods (wasn't the case with Xtend 2.15.0). In our build, for example, these make around 1.5k warnings which clutter the log.

RATIONALE: if someone is interested in optional warnings of some type, it is usually sufficient to see them in the IDE only (in a CI build they are usually overlooked, anyway). On the other hand, when unwanted warnings - that were deliberately turned off in the IDE by the user - show up in a CI build, they can become very cumbersome.

[1] https://bugs.eclipse.org/bugs/show_bug.cgi?id=481920

Start by locating the xtend-maven-plugin handling for Xtend warnings and compare it with warnings configurable through .settings/org.eclipse.xtend.core.Xtend.prefs. Reproduce the unnecessary-modifier warnings described for Xtend 2.16.0, then verify that optional warnings no longer clutter Maven output while required warnings remain visible.