Updater (Swing) Manage Update Sites: show the 'active' column

In one of the next commits, we will show inactive update sites, too.

While at it, use the new and enhanced UpdateSite class internally.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (Swing) Manage Update Sites: use update site list from the Fiji Wiki

The Fiji Wiki has the page

	http://fiji.sc/List_of_update_sites

which all ImageJ users are welcome to add their update sites to. To make
things really convenient, we now offer that list in the 'Manage Update
Sites' dialog of the Updater's advanced mode.

This addresses http://trac.imagej.net/ticket/1842.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (Swing) Manage Update Sites: make list larger

Now that we populate the list from the update sites publicized via
the Fiji Wiki, we need more screen estate.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater (Swing) Manage Update Sites: show description/maintainer
in tooltips

The description and maintainer information from the Fiji Wiki should
also be made available to the interested user...

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

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

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