Move TrakEM2 and related projects to their own parent POM

In line with the recent push for Reproducible Builds and a better
structuring, let's let TrakEM2 manage its Bill of Materials in its own
POM.

For further rationale, see http://fiji.sc/Reproducible_builds and
http://fiji.sc/Bill_of_Materials, respectively.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>