Fixing styling issues with CellToolbar.

* Width of CellToolbar was off on Linux Chrome+Firefox.
* Minor styling cleanups.
gshow
Further cleanup for celltoolbars.
Minor tweaks to the css to enable single cell hide/show.
Decoupling the celltoolbar select UI from CellToolbar.

The select menu for celltoolbar presets should live entirely in
the maintoolbar code - it was 1/2 in CellToolbar itself. This
fixes this issue using events and an additional method on
CellToolbar (list_presets).
Removing unneeded ui-widget class.
Merge pull request #2789 from ellisonbg/celltoolbar-style

Fixing styling issues with CellToolbar.