Add a WebDAV-based uploader for the ImageJ updater

The idea is to allow more contributors to offer their plugins and
extensions more easily.

To provide a WebDAV-based update site with Apache, make a directory
(preferably in the webspace) and add a file looking roughly like this to
Apache's conf.d/ directory:

	<Location /update>
		DAV on
		AuthType Basic
		AuthName "ImageJ update site"
		AuthUserFile /etc/apache2/webdav-uploaders.db
		Options +Indexes
		<LimitExcept HEAD OPTIONS GET>
			Require valid-user
		</LimitExcept>
	</Location>

Then, initialize the password file like this:

	sudo htpasswd -c /etc/apache2/webdav-uploaders.db rasband

Finally, make sure that the directory referenced in the <Location> tag
is read/writable by the Apache process (typically www-data), and restart
Apache gracefully:

	sudo chown -R www-data.www-data /var/www/update
	sudo apache2ctl graceful

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make test-jars in all projects

This is a useful feature to be able to re-use test classes in dependencees.

Suggested by Curtis Rueden.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Refactor the updater test

The helper methods can now be reused in dependent projects.

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

No need for a 'German' method name, and no need to require a progress
instance, no need to carry around ijRoot and webRoot when a
FilesCollection carries both...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a unit test for WebDAV

This test is only activated if a test WebDAV web space is defined via
the webdav.test.url, webdav.test.username and webdav.test.password
system properties. The web space pointed to is expected to be empty.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix compiler warnings in ij-updater-ssh

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