Editor: More space through tab panel

The west panel of the editor currently shows its items as an accordion menu. The headers of all subitems are permanently shown and take up space which would be better suited for the subitems themselves. This is primarily a concern when VisualReview is active as the editor then has limited height.

• Use a tab panel instead of an accordion.
• Add icons and tooltips to the tabs.
• Show at most the header of the current subitem, or none.