MiniMaven: handle  in the imagej.app.directory property

This is required for Fiji to build properly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Let MiniMaven clean up multiple versions of dependencies, too

If a project has a dependency that is present in the jars/ directory
in multiple versions, let MiniMaven delete the non-matching ones,
even if the dependency's correct version was already up-to-date.

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