Blender Git Loki

Cleanup: refactor filelist filter functions

Perform a few cleanups:

- Add documentation to explain what returned bools mean.
- Early returns so that flow is clearer and some checks on `is_filtered`
are no longer necessary.
- Split up `is_filtered_file` and `is_filtered_id_file`, so that they can
reuse common code, and such that the different filter checks they
perform can be separated from each other.

The latter is done not only to reduce code duplication, but also as
preparation to fix the asset browser filtering. For that, it helps when
the "filter by file name" and "filter by file type" parts are separate.

No functional changes.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D12991