Make Maven version an explicit prerequisite

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Format the source code of the copy-jars mojo

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

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Refactor the copy-jars mojo

We are about to introduce a mojo that installs artifacts *without*
needing a Maven project. This mojo shares a lot of the business logic
with the copy-jars mojo, so let's have a shared base class for both.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add the install-artifact mojo

This mojo is similar to maven-dependency-plugin's 'get' goal, except in
two respects:

1) It copies ImageJ 1.x plugins into the plugins/ subdirectory, all
   other artifacts into the jars/ subdirectory, of the ImageJ.app/
   directory specified by the 'imagej.app.directory' property.

2) It includes the dependencies (both direct and transitive) in the
   task.

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