avoid raw Types when dealing with scene2d Cell

The integration of scene2d ui into libgdx seemed to do something with generic types that broke the API for Kotlin (and other JVM languages that don't support raw types). In short, it made chaining impossible which meant lots of redundant casting in a new statement, which is a pain.

This PR fixes that.

I did some rudimentary testing with existing scene2d ui apps, seems to not have broken anything.