Introduces a new `AssetRepresentation` type, as a runtime only container
to hold asset information. It is supposed to become _the_ main way to
represent and refer to assets in the asset system, see T87235. It can
store things like the asset name, asset traits, preview and other asset
metadata.
Technical documentation:
https://wiki.blender.org/wiki/Source/Architecture/Asset_System/Back_End#Asset_Representation.
By introducing a proper asset representation type, we do an important
step away from the previous, non-optimal representation of assets as
files in the file browser backend, and towards the asset system as
backend. It should replace the temporary & hacky `AssetHandle` design in
the near future. Note that the loading of asset data still happens
through the file browser backend, check the linked to Wiki page for more
information on that.
As a side-effect, asset metadata isn't stored in file browser file
entries when browsing with link/append anymore. Don't think this was
ever used, but scripts may have accessed this. Can be brought back if
there's a need for it.
We would first invoke the dragging, and then set the drag data (like the
ID or the dragged modifier), so the `wmDropBox.on_drag_start()` handler
wouldn't be able to access this. This broke dragging some IDs from the
Outliner, noticed in D15333.
It's now possible to first create/request drag data, extend it, and then
invoke the actual dragging. The normal function to start dragging
returns `void` now instead of `wmDrag *`, so the drag data can't easily
be modified after starting anymore.
Moves code for managing dragging data from buttons to a separate file.
This way all this closely related code is in one location, making it
easier to see how it all relates, and easier to find.
