Enforce the FijiClassLoader via Javassist

Our previous method was embedded in Fiji's User_Plugins plugin which
is a bit backward, but worked. But it is better (and shorter, to
begin with) to patch ij.IJ#getClassLoader() directly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a plugin to print stderr/stdout also to the Log window

This plugin is not intended to be run by the user, but by Fiji,
first thing after ImageJ 1.x has been initialized.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Start the stderr/stdout redirector right at the start

As suggested by Curtis Rueden, we can (and should) show the stderr and
stdout stream in the Log window, to give users a better chance to report
(or even fix) problems that are typically only reported to stderr.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Johannes to Git: Ignore IntelliJ files, please

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