Refactor out @Attr annotation usage

[hinerm]

The Attr sub-annotation in Plugin annotations is messy and was intended to be a last ditch effort.

In the primary use cases of Attr in SCIFIO, Filters and Translators, we really don't need the information BEFORE an instance of the corresponding class is made.

So we should refactor Filters and Translators to use API instead of static fields and Attrs, which is also then extensible.

This should be done in tandem with #20.

Also, Translators could then be SingletonServices and SingletonPlugins.

This change should also allow us to fully remove Annotation usage from tests (e.g., FilterTest).

[ctrueden]

FWIW, I did this just now for Tool plugins in ImageJ2. See imagej/ImageJ@3fc174b for details.