Don't set absolute_top from text block

This is a slightly speculative branch in response to @PragTob's totally valid point that it's weird for the `TextBlock` class to be setting `@dsl.absolute_top` just to get those numbers back to the slot positioning.

The change here provides a fully-allowed override that elements can give to meddle with the slot's `current_position` after it's done all its modification. `TextBlock` is currently the only thing that actually needs it.

This breaks rather a lot of specs, but wanted to get commentary on whether this is preferable before I do the grunt work of patching up all the text fitting specs to behave correctly with regards to this change. What do you think @PragTob @wasnotrice @KCErb? Is this worth applying?