Teach ImageJ 1.x to be careful when adding images into the Window menu

When ImageJ 1.x adds images to the Window menu, it does not care whether
there were already some plugin installed or not; it always appends the
new menu item. Instead, it should insert the correct position.

This fixes https://github.com/imagej/imagej-legacy/issues/68.

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