Allow bootstrap.js to be run with a GUI but without ImageJ 1.x

The original intent of this script was to allow the ImageJ updater to run
in ImageJ 1.x, but it was extended to bootstrap ImageJ and now we will
adjust it so it can be used to fix non-working installations.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bootstrap.js: dispose the temporary output frame

When starting the bootstrap script in GUI mode without ImageJ 1.x, we
need to provide an output panel. After the updater started correctly,
we no longer need it.

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