Removing CurrentPosition#moving_next

For text flowing, the current position during slot positioning started
to track whether if it was moving to the next line. Text fitting used this in
certain available space calculations.

Upon further investigation, the text block has been placed, so we have the
values necessary in the DSL's absolutes to determine what's happening. We still
use current_position.next_line_start, but don't require an explicit new flag.
Make better use of parent's existing methods
Fix absolute_top of element below next line start

I'd previously removed a <= on the next_line_start to absolute_top comparison,
thinking it wasn't necessary. That's the case when we're flowing naturally,
but when we bump the absolute tops of things further down, we do need the <=
still (for instance, samples/sample4.rb)
Reverting from using absolute_right

Top-most app container doesn't have absolute_right on it (yet), so we need to
keep using the absolute_left + width instead for the time being.
Merge pull request #557 from shoes/removing_moving_next

Removing CurrentPosition#moving_next