Use layout-fragment heights, deprecate typingLineHeight, fix extra-line metrics

- Switch to layout-fragment and selection heights; deprecate typingLineHeight
- Simplify FB15131180 workaround via extraLineFragmentAttributes
- Ceil default line height and remove implicit stTypographicBounds variable
- Add secondary editor window on launch; rename/refactor primary VC; extract
  attachments
- Remove unused lastViewportBounds tracking from STTextView
- Suppress relayout when viewport is vertically contained; add upward prep band
- Note CalculateDefaultLineHeight is not really used