Blender Git Loki

UI: Indicate asset data-blocks with an icon in Outliners & search menus

It's useful to easily see which data-blocks are assets and which not. So just
like we usually show the library linking/override icons, we show the asset icon
there (these are mutually exclusive data-block states).

Uses the `'MAT_SPHERE_SKY` icon, which wasn't used before (except by an
add-on!) and is sorta fitting, but not quite. We should either change this one
or add an own asset icon. Meanwhile this isn't too bad :)

Also adds an internal macro to check if a data-block is an asset, consistent to
how we do it for libraries and library overrides.