Safeguard addToJarRecursively()

We cannot know for sure that the source directory exists at all... (for
example, Fiji's Sample_.jar has only resources, no sources).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Avoid creating a File object for the same path twice

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix generating .jar files including sources

We used to include only the .java files, and not even in the
src/main/java/ directory structure at all. Let's include the pom.xml and
the resources, too, and let's put things neatly so that unpacking the
.jar file will contain a valid Maven project -- in addition to the
compiled .class files.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'include-sources'

This topic branch fixes the generation of .jar files including the
sources required to rebuild the project.

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