Fix class members in Outline

Class members were displayed as top-level items (not indented and not highlighted when the class was hovered over).

This also further streamlines the relationship between the items-to-scroll-to vs. items-to-add-to-top-level-list hierarchies.
Fix highliting of classes in outline + clean up outline CSS
Merge remote-tracking branch 'upstream/master' into resizable-outline-and-better-button

Conflicts:
	ide/app/lib/outline.dart
	ide/app/spark_polymer.css
Restore returns for outline items created by _create* and _add* methods
Merge pull request #2618 from ussuri/resizable-outline-and-better-button

Fix class members appearing as top-level elements in outline