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.
So unfortunately `style_init` needs `@app` to be set, which is the only reason why we pass it in. That is needed for the `App` level styles that are defined. Normally `Text` has an accessor to get app from the text block, but this is only set after the creation of an individual part... so yeah we need the direct reference.