Sidebar widget

A number of our apps have custom (and mostly not very good) sidebar implementations, would be cool to have a proper widget that supports the most common use cases.

Features I think we'd need (though maybe not all of them from the start):

• Icons
• Action buttons (e.g. unmount drive)
• Status icons (e.g. git status)
• Sections with and without headers
• Counter bubbles (e.g. message counters)
• Secondary click/long press context menu
• Collapsible sections
• Drag and drop re-ordering within sections
• Drag and drop moving between sections
• Nested sub-sections

I'm not sure about/needs more thought:

• Selection mode
• Controls other than buttons on the right (e.g. switches)

Potential use cases include:

• Nautilus (needs sections w/o headers, icons, action buttons)
• Geary (needs sections, nested sub-sections, icons, counters)
• Fractal (needs icons/avatars, DnD moving between sections)
• Libadwaita demo (needs sections, icons)
• New mode in HdyPreferencesWindow for apps with a ton of preferences (needs sections, icons)

In terms of visuals, these mockups are a start: https://gitlab.gnome.org/Teams/Design/app-mockups/-/raw/master/patterns/patterns.png