When updating the updater, do not loop infinitely

When the updater updated itself, it mistakenly thought that the files
were not yet downloaded and tried downloading them in an infinite loop.

The trick, of course, is to remember that we are in the middle of
updating the updater itself, and disable the check.

This issue has been reported by Claudio Collinet in
http://fiji.sc/bugzilla/show_bug.cgi?id=763 and on the mailing list.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Use the thread name as marker that we're updating the updater

The previous method -- to use a system property -- was too error-prone:
it depended on the thread running to its end to reset the property.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Let the imagej.dir property override the ij.dir property

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: make file table's column handling safer

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make sure that *some* files are shown in advanced mode

When the file table is empty and the user switches to advanced mode,
experience tells us that they are looking for a specific file. The
best idea is to show all the files, and already sort them by status,
so that the user only has to type in part of the file name into the
search box.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Bump to next development cycle

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