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
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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
