The catalog tree is a unit on its own, and should be tested separately.
This makes the testing files smaller and more focused, which can help
maintaining them.
This manages setting up asset library directories for testing, which is
useful for testing multiple asset library related compontents. So move
it to a common header. No reason to squeeze everything into one file
then.
Adds a new `source/blender/asset_system` directory and moves asset
related files from BKE to it. More asset related code can follow
(e.g. asset indexing, ED_assetlist stuff) but needs further work to
untangle it. I also kept `BKE_asset.h` and `asset.cc` as is, since they
deal with asset DNA data mostly, thus make sense in BKE.
Motivation:
- Makes the asset system design more present (term wasn't even used in
code before).
- An `asset_system` directory is quite descriptive (trivial to identify
core asset system features) and makes it easy to find asset code.
- Asset system is mostly runtime data, with little relation to other
`Main`/BKE/DNA types.
- There's a lot of stuff in BKE already. It shouldn't be just a dump for
all stuff that seems core enough.
- Being its own directly helps us be more mindful about encapsulating
the module well, and avoiding dependencies on other modules.
- We can be more free with splitting files here than in BKE.
- In future there might be an asset system BPY module, which would then
map quite nicely to the `asset_system` directory.
Checked with some other core devs, consensus seems that this makes
sense.
