Arrow button style class
A common pattern of a smaller linked button which opens a popover/menu. Having this in Adwaita will bring consistency.
Pattern found in:
- Nautilus (view mode)
- Builder (run button)
- Glade (open button, normal size)
- Gedit (open button, normal size)
- LibreOffice and others non-GTK apps
Similar to #1273 (closed).