I'm trying to move away from general files with lots of things in them, and instead have many small & focused files. I find that easier to work with since everything has clear responsibilities, even if there is some minor overhead in managing all these files. I also try to differentiate more clearly between public and internal files. So source files and internal headers are in a `intern/` sub-directory, public functions are in a number of headers one level higher. For convenience and to make this compatible with our existing general headers in `editors/include`, I made the `ED_asset.h` there include all these public headers. This is of course a bit of an experiment, let's see how it works in practice. Also corrected the name of `ED_asset_can_make_single_from_context()`.