About ImageJ: fall back to a signed, 16-bit gradient

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
About ImageJ: remove unnecessary restrictions on the images

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
About ImageJ: shorten the long hex string if possible

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'master' of git://github.com/imagej/imagej