Blender Git Loki

Add option to link assets on drag & drop

Note: Linking in this case as in link vs. append. Easily confused with linking
a data-block to multiple usages (e.g. single material used by multiple
objects).

Adds a drop-down to the Asset Browser header to choose between Link and Append.
This is probably gonna be a temporary place, T54642 shows where this could be
placed eventually.

Linking support is crucial for usage of the asset browser in production
environments. It just wasn't enabled yet because a) the asset project currently
focuses on single user, not production assets, and b) because there were many
unkowns still for the workflow that have big impact on production use as well.
With the recently held asset workshop I'm more confident with enabling linking,
as design ideas relevant to production use were confirmed.

Differential Revision: https://developer.blender.org/D11536

Reviewed by: Bastien Montagne