Auto load missing assets from assets folder

[felix-ht]

If one adds a symbolLayer and uses an image from assets the icon - it does not show up. To fix this we should handle the callbacks for missing assets to the native side.
Follow up on #779

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[felix-ht]

fixed by #929