When cleaning, also remove matching files from ${imagej.app.directory}/

It is counterintuitive when installing removes other versions from the
ImageJ.app/ directory, but cleaning does not even remove the artifact
which would be installed.

So let's fix that bug.

Reported by Curtis Rueden in:

	https://github.com/scijava/minimaven/issues/6

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a regression test for MavenProject#clean(ijDir)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'clean-ij-dir'

This branch addresses https://github.com/scijava/minimaven/issues/6.

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