Group fields into panes

[spikelynch]

Sidebar has its own config, each pane has a list of fields

[spikelynch]

Tidying up Claude's first draft of this:

• make a new Popup section (under "Linked items")
• Move the sidebar toggle to "Popup"
• Make the sidebar pane fieldset look the same as the popup properties
• when the sidebar is off, it should hide all but the first pane of properties (ie a normal popup is a single pane)