Move Wicket viewer under core.

This topic was discussed on dev@isis.a.o, no objections [1]

For this release, do NOT change the groupId and artifactId... to avoid disruption to the user community. Will change these instead in 1.9.0.

The primary benefit of this change is for the release process.

[1] http://isis.markmail.org/thread/ox3i6arna4n7gz3v