Renamed relative Actor methods: translate, rotate, scale, size

For consistency, becomes: moveBy, rotateBy, scaleBy, sizeBy
May have conflicts with Actions static import, eg you'll need to use "Actions.moveBy".

Also, this should have been a separate commit but accidentally got stuck in here: Table background is now drawn after the transform is applied, so the background is transformed like the children.
Javadocs
Added Container, which is similar to a Table with one cell, but more lightweight.

#1336
Fixed some issues with Table#drawBackground being called inside the table's transform.