TextBlocks have #parent not #parent_text, refers to the same though

We introduced `parent_text` for `Text` elements to refer to their parent text_block. Turns out in Shoes3 this is just called `parent`, how we usually referred to the slot around it. :)

This introduced the problem in #979 - where a link did get handed a parent which was a slot and was conflicting.

These elements now don't have a reference to the slot anymore, but they don't need it. Should we need it, we could change it through the text block.