Make sure that ImageJ 1.x processes plugin .jar files alphabetically

To ensure a consistent ordering given the same installed plugins, we
*must* sort the list of plugin .jar files alphabetically. Otherwise,
file system-specific details (or worse, order of installation)
juggles everything out of order.

This fixes https://github.com/fiji/fiji/issues/79 too because Fiji only
has three plugin .jar files that install menu items into the Help menu:
Bug_Submitter.jar, Fiji_Plugins.jar and Fiji_Updater.jar. And the last
of the three is the one that is supposedly processed last, making sure
that the last two entries into the Help menu are the ones most likely of
interest: Fiji Wiki and Update Fiji.

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