Command-Line: use imagej.dir property

The imagej.dir property is now the principal property to tell where
ImageJ.app/ (or Fiji.app/) live. For backwards-compatibility, we fall
back to ij.dir and fiji.dir, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Command-Line: allow partial initial installation

When there is not even a downloaded db.xml.gz yet, we might still
want to install only a part (e.g. the updater).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Do not record versions for dependencies

This addresses https://github.com/fiji/fiji/issues/42.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Auto-enable the Fiji update site

This addresses https://github.com/imagej/imagej/issues/55.

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

This branch collects assorted fixes for the updater's command-line
interface.

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