Let `BlockLike` functions accept `&mut Ui`

[Hexorg]

To add syntax highlighting and clickable elements into a block - the BlockLike trait functions should accept &mut Ui argument instead of returning string references.

[Hexorg]

I'm looking at your code. I'll try to make a pull request. This seems like a hard change, because you need to know block sizes to make the layout, but if you use &mut Ui like I suggested you can't compute layout ahead of time. I'm trying to see if I can port your layout function to layout "just in time" as we're drawing blocks.