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.