Blender Git Loki

Assets/UI: Resolve major asset view UI template limitation

Before this, all asset view templates showing the same asset library
would show the same assets, even if they should show different ID types.
That was a major limitation since the design did forsee that this
template can be put anywhere in the UI to display various sub-sets of
assets.

Initially I did the ID type filtering close to the asset-list reading,
because I wanted to optimize reading so that we would only actually read
asset information from disk of the ID type to be shown. But this will be
quite complex and I'm not sure if I'll get to work on this anytime soon.
So this commit moves the filtering to the template display level solving
this limitation.

Note: This also adds the code to filter by tags, together with the ID
type. But it's not actually used anywhere yet.