When a preset is activated fire "preset_activated" event.
On "preset_activated" event, update the value of the maintoolbar select element that lists celltoolbar presets.
When a preset is registered after notebook is loaded, active it if needed.
Make sure that celltoolbars are hidden
when a notebook without "celltoolbar" entry in metadata is loaded.
Merge pull request #5605 from AlbertHilb/CellToolbar

Two cell toolbar fixes.