Disable implicit layer animations in gutter views and improve gutter width handling

- Override animation(forKey:) to return nil in several gutter view classes to
  prevent unwanted implicit animations when view properties change.
- Use CTLineGetTypographicBounds instead of CTLineGetBoundsWithOptions for more
  accurate sizing in STGutterLineNumberCell.
- Initialize STGutterLineNumberCell with a frame that reflects its text size.
- Refactor gutter width adjustment logic to occur only when necessary and
  prevent layout regressions during scrolling or layout.
- Clean up unused CATiledLayer usage in STGutterSeparatorView and preserve
  graphics state when drawing the separator.