Highlight context menu items at the top of the 2D/3D viewports

Created by: Calinou

This makes it easier to notice that some menu items only appear when specific nodes are selected.

This change applies to both 2D and 3D editors, including both plugin-based menus and the hardcoded 2D layout/animation contextual menus.

I'm quite sure this addresses an existing issue/proposal (if not several), but I can't find it anymore.

Preview

A blended version of the editor theme's accent color is used for highlighting.

Dark theme

Light theme