Move TextBlockCursorPainter usage

This commit simply moves where we call into the TextBlockCursorPainter to a
location more appropriately located with other drawing for text blocks.

Later work will revise the TextBlockCursorPainter to make better use of what
FittedTextLayoutCollection has to offer.
Use FittedTextLayoutCollection's range methods

The cursor painting previously had its own custom logic for determining
which of the flowing text layouts we were going to position the cursor
in. This logic is a subset of what FittedTextLayoutCollection already
does for spanning links, so we now call the collection with our layout
question.
Pass DSL to the TextBlockCursorPainter

We have it at the point of instantiation, and it makes the upcoming
changes easier to apply if it's already an ivar on the class.
Move relative text positioning to FittedTextLayout

Cursor relative text positioning is really better suited to the
FittedTextLayoutCollection class that deals with multiple flowed
layouts.
Minor shuffling of methods in cursor painter
Move cursor height to FittedTextLayoutCollection
Removing unnecessary cruft
Rename dsl to text_block_dsl for clarity
Merge pull request #721 from shoes/cursor_revision

Revise TextBlock cursor to be more like other text functionality. Changelog: improvement