Remove dependency on siblings

We can get all the necessary positioning information we need from the current
position we now get passed.
More accurate positioning in subslots

Current position in in absolute terms so we need to reconcile that when using
the parent width.
Don't bother passing text_block, use ivar
Fixing and refactoring assignment of absolute_top

When we are looking to set the absolute_top for advancing the current_position,
this has to take into account 1) the height of the second layout 2) the current
y position, 3) the height of the last line of text (which our next positioning
needs to align with the top, not the bottom of), and 4) the spacing on the
layout.

Whew. With all that right, things line up. :fingers_crossed: