Temporarily bump the imagej-updater dependency to a SNAPSHOT version

There are a few things we want to develop that require changes in
imagej-updater as well...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Save db.xml.gz even when just enabling/disabling update sites

Since db.xml.gz contains the information which update sites we follow,
we might need to write it even if no files were updated.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Avoid using the deprecated AppUtils class

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Handle file status/action correctly after disabling an update site

This is the big change to use the just-introduced API that is supposed
to help us when disabling an update site.

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