Backport PR #5605: Two cell toolbar fixes. 1. When a new notebook is loaded we should update the state of the ``select`` element listing the cell tollbar presets in the main toolbar, so that its value reflects the ``celltollbar`` metadata entry of the notebook. 2. When a new notebook without ``celltoolbar`` metadata entry is loaded we should make sure that the cell tollbars are hidden. In particular, that is needed when you are working to a notebook with a certain cell toolbar preset and then you revert the notebook to a state with no cell toolbars.