Updater (Swing) Manage Update Sites: simplify 'Add' handling

Since we can have inactive update sites in the site list now, let's
use that mechanism for manually adding update sites.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: add url and upload directory setters to the UpdateSite class

They really wanted to be setters all along because they require a
trailing slash.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: move the UpdateSite class into its own file

While at it, enforce the use of getters and setters (as seen with
the URL and the upload directory, it is a good idea to have setters
to be able to enforce certain formats.

Since this is an API-changing commit, it requires fixes in all uploaders,
and in Fiji's Fix_Updater.js, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (Swing) Manage Update Sites: only update String values after editing

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (Swing): reconcile activateUpdateSite() and readUpdateSite()

These two methods really wanted to do the same to begin with.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
UpdateSites: rename the 'sshHost' field to 'host'

It is really no longer limited to an SSH host.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (Swing) Manage Update Sites: optionally add a personal update site

Now that every ImageJ user can offer their own plugins/macros via their
own update site at http://sites.imagej.net/<username>/ (where <username>
is the name of a Fiji Wiki account), we can make it much easier to add
that site.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MediaWikiClient: add #userExists(String)

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

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