Add File Browser Link/Append option to show only assets

Support "Author" metadata field

Merge branch 'master' into asset-metadata

Add access to asset-data from ID

Remove asset data-block again, back to ID.asset_data design

So rather than having an asset data-block to reference data-blocks, let
the data-blocks reference asset-data.

After talking to Brecht, it seems that this approach is just fine. I previously
added the data-block type because I saw some benefits (faster & simpler asset
metadata reading, non-data-block assets, better file-browser integration,
simple remapping of referenced data-blocks), but there are ways to do the same
without the data-block. It's a bit more complicated that way, but still very
doable.

Add custom tag support to assets

Tags are basically a list of strings. We could optimize these in future if
needed. We could also easily add colors or icons for individual tags (e.g. like
on developer.blender.org).

Note that there's no UI for this yet. Functionality is available via Python
though.

Support arbitrary descriptions for assets

Simply have a dynamic string in the asset data-block. That can be set and
updated at any point.

Support preview thumbnails for new asset data-blocks in file browser

Assets have to own a copy of the referenced data-block's preview for this to
work. We *could* do without that, but it complicates partial reading of
data-blocks for the file browser (which may be addressed with further work
though).

Use new Asset data-block type for "Create Asset" operator

AssetData is unused now. Just keeping it in case it's useful later (to avoid
having to do the monkey work to add it again).

Merge branch 'master' into asset-metadata