Cleanup: Move reorganize asset files

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()`.

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