Switch to Modern Mode: Make sure to use the *correct* context class loader

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ensure the correct context class loader already in LegacyInitializer

This is actually "more correct" than the previous commit, but let's keep
both, just for safety reasons (somebody enterprisey might reset the
context class loader between LegacyInitializer and Switch to Modern Mode).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Defensive programming in DefaultLegacyService

When run in ij-legacy's tests, we cannot assume that there is a default
user interface at all.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Temporarily bump the version of the ImageJ 1.x dependency

ImageJ 1.48r has a critical fix regarding redirecting error messages that
we are about to use.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Verify that Switch to Modern Mode works

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

This topic branch addresses some Switch to Modern Mode issues with Fiji.

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