Add warnings about invalid characters in filename
Description
As noted in #809 (closed), we need to declare a set of characters that make up a valid element name. Based on the discussion started in https://mail.gnome.org/archives/buildstream-list/2018-December/msg00045.html, implement the necessary changes to the loader.
Changes proposed in this merge request:
- _loader/loader.py: Refactor warnings about element names in one method
- _loader/loader.py: Add warnings about invalid characters in filename
- doc: Add note about valid element names
This merge request, when approved, will close: