Add focusable variants of widgets

[fourlastor]

This PR adds a set of actors to the scene2d extensions that mimic scene2d's default widgets, but automatically declare themselves to be focusable on setStage

[MrStahlfelge]

Can you add a paragraph to https://github.com/MrStahlfelge/gdx-controllerutils/wiki/Button-operable-Scene2d ?
(You don't have access, but please add a comment here and I will copy it to the wiki entry when merged)

[fourlastor]

Sure thing! I'd place this after the ControllerList description.

Focusable actors

Most of the scene2d widgets have been extended to automatically call ControllerMenuStage#addFocusableActor. They follow the original naming, prepended by Focusable (FocusableButton, FocusableImage, and so forth).

[fourlastor]

Would it be possible to release a new version with these?

[MrStahlfelge]

Sorry for not replying so long. I wanted to release a new version, but at the moment the project is not compatible with Java 11. Gradle needs to get upgraded and this needs other changes to the build files as well. As I am really busy right now and did not follow the general changes on libgdx builds at the moment, I would really appreciate a PR updating the build environment so that I can do a release quickly. Thanks :)

[fourlastor]

Ask and you shall receive 😁 #29

[MrStahlfelge]

Great, thanks. 2.4.0-SNAPSHOT is available now, release builds do not work (yet), more in the PR.