animation: Use appropriate spinner asset in light variant

Ideally we would replace the sliced-image based animation with a themed process-working-symbolic icon and rotate it, so the spinner simply picks up the current foreground color.

Unfortunately the repeat-count property does not work for rotations, so to fix the broken spinner in the light variant in the meantime, include assets for both variants and swap them out at runtime.

Not everything in the light variant is actually light (overview, OSDs, ...), so use a simple heuristic on the text color to decide which asset to use.

Close #6783 (closed)