Text cursor tidy

Revisions in the text cursor painting based on feedback from @PragTob on #493.

* Removing special logic that assumed order of checks for single layouts
* Clearer names in cursor positioning logic
* Moving calculation of cursor height (which was only consumer of an odd method on `FittedTextLayout`) into the cursor painter itself

Still feeling my way out on what to PR and what to just do on master... How would you have gone about this set of changes @PragTob?