Adwaita: Improve popover.menu styling

Based on the mockup and the discussion in #1824 (closed), I polished the popover menu styling.

Visible changes are:

• Flatten circular buttons in popover menu
• Remove horizontal padding from popover menus
• Remove roundness from menu items
• Use the existing menu sizing for popover menus
  • EDIT: Adjust the sizing to match the mockup
• Make check boxes and radio buttons symbolic icon style
• Fix visual glitches on inline buttons

CC @aday @jimmac