Support gdx-controllers extension

[nicolaichuk]

In this file

https://github.com/libgdx/libgdx/blob/master/extensions/gdx-controllers/gdx-controllers/src/com/badlogic/gdx/controllers/Controllers.java#L102

if (type == ApplicationType.WebGL) {
			className = "com.badlogic.gdx.controllers.gwt.GwtControllers";
}

and when i compile program with

-keep class com.badlogic.gdx.controllers.** { *; }

and open in browser. I see in js browser console:

....
ClassNotFoundException - com_badlogic_gdx_controllers_gwt_GwtControllers
....
ReflectionException - Class not found: com.badlogic.gdx.controllers.gwt.GwtControllers
....
GdxRuntimeException - Error creating controller manager: com.badlogic.gdx.controllers.gwt.GwtControllers
....

Be well if gdx-dragome-backend supported controllers api like gwt

https://github.com/libgdx/libgdx/tree/master/extensions/gdx-controllers/gdx-controllers-gwt