POM: update child projects to pom-scijava 1.126

This change also updates imports as needed for the new version of
ImageJ (2.0.0-beta-7.6) which has been pinned (some classes moved).

It also performs some minor tweaks to POM whitespace and comments.
CreateANewPluginType: stop using SortablePlugin

The class is now deprecated, having been effectively renamed to
AbstractRichPlugin.