Add scrollmanager
Make slideshow toolbar add classes to modified cells.
Fixed bug
Added smooth scroll to replace std browser behavior.
Add ScrollManager to global ipy namespace in main.js instead of,
in the bottom of the scroll manager js file.
Added scroll mode selector,
fixed rebase conflicts.
Added heading and slideshow scroll managers
Fixed rebase indents
Removed ScrollManager selector combo.
Address review comments
options bug fix
Added comments clarifying scroll units
Fixed dumb typo,
forgot to update scrollmanager ref to scroll_manager.
Added what's new
Scroll to all heading levels <= target level.
Set heading scroll manager as default scroll manager.
Make the default the single page scroller.
Make HeadingScrollManager scroll to heading elements, not cells.
Merge pull request #5844 from jdfreder/scrollmanager

'Paul' scroll feature