Depend on ImageJ2 directly

Earlier, we tried not to do that because the Fiji_Updater.jar was not
supposed to have *any* dependencies -- for backwards-compatibility
with the times when the Fiji Updater knew how to update itself, but
did not expect any dependencies (and therefore, failed to update or
install them).

However, we *can* cope with version skew by catching exceptions
thrown in the absence of imagej-updater.jar. And that's what we're
going to do: by default, use ImageJ2's updater properly. And
falling back to hackery when this is not possible.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a basic adapter for the ImageJ2 updater

The only reason why this functionality needs to live in a separate
class is to be able to catch exceptions properly when the ImageJ2
updater is not actually in the class path.

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

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Use the ImageJ2 updater adapter

This is a much cleaner way to depend on the ImageJ2 updater.
Unfortunately, we have to have two different levels of fall backs:
The ImageJ2 updater *just* moved to a new package, and we need to
fall back to the old package name if it cannot be found in the new
spot. And if it cannot even be found in its old home, we still need
to fall back to calling http://update.imagej.net/bootstrap.js
either directly via Javascript, or opening it via File>Import>URL...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'imagej-updater'

This topic branch prepares the Fiji Updater for the impending release
of ImageJ2's updater in the imagej-updater.jar and imagej-ui-swing.jar
artifacts. This coincides with the move to stand-alone source code
repositories, and at the same time with the move to a new package:
net.imagej.updater (it used to be imagej.updater, but that does not
*exactly* conform with known best practices, so we changed it).

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