Support dependency injection for Translator plugins UI class

UI classes are a relic from the Drupal 7 ctools plugin system this is originally based on. I think the most feasible way forward instead of more workaround is to just deprecate UI classes and inline them into the main plugin class. In some cases, that will result in large classes, but this is the common approach for plugins, from blocks to field types, they both manage the data and their settings forms.

We could deprecate that in 8.x-1.x, and allow plugin classes to directly implement the UI interface and in a future 2.x branch, merge and require it.