MiniMaven: let system properties override settings from pom.xml

We want to be able to overrule the pom.xml settings by passing
command-line arguments...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add Javadocs to some MiniMaven classes

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven: support the -D<key>=<value> command-line option

While at it, refactor the usage printing, and add the 'final' keyword
in a couple of places.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Move the code to work with versioned file names into FileUtils

We need the very same code in MiniMaven, so let's share it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix a few compiler warnings in the updater code

... and add a couple of Javadocs, while we're at it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven: offer to install the built artifact and its dependencies

This replicates the functionality of the imagej-maven-plugin: a .jar
file and its dependencies can now be copied to an ImageJ.app/ directory
structure; just set the imagej.app.directory property either in the
pom.xml file or as a system property to ask for the install step.

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