ij-legacy: handle IJ.runPlugIn("ij.IJ.init", null)

In headless mode, we need a way to force ImageJ to initialize itself
even without running a macro file and certainly without creating an
instance of ij.ImageJ (which would fail because ImageJ is an instance
of an AWT Frame which cannot be instantiated in headless mode).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ij-legacy: initialize IJ1 properly even in headless mode

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ij-legacy: find legacy commands even in headless mode

They simply won't have any menu path in headless mode, is all.

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

Thanks Jennasee Jenkins!

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