Add TrackMate artifact to the Fiji distribution

Our current approach is to explicitly list artifacts for inclusion in
the Fiji distribution, by specifying them as external dependencies.

Let's do this with TrackMate, since nothing in src-plugins explicitly
depends on it, but it should definitely be included in the Fiji build.
gitignore: relocate the TrackMate exclusion

It is now a dependency pulled in by MiniMaven, and no
longer a direct artifact of the multi-module build.