Add non-image-specific services from ImageJ core

This includes the module and command frameworks, including widgets
(module preprocessing that handles inputs) and displays (module
postprocessing that handles outputs).

It also includes the menu, options, platform, script and tool services.

Migrated from:
https://github.com/imagej/imagej/tree/imagej-2.0.0-beta-7.9/core/core/src/main/java/imagej
ContextCreationTest: expect new services, too
Remove imagej-isms from javadoc
SciJavaPlugin: add core plugin type descriptions

This was migrated from the javadoc of ImageJ's ImageJPlugin.