Review, amend and update the XCF file format spec and parasites.txt

XCF spec:
- Update to GIMP 2.8.10
- Clarify role of file formats in the save-vs.-export-context
- Rearrange outline
- Collect infos on basic concepts in one chapter
- Add table of contents
- Add File format version history
- Add note on image size
- Add open questions and TODOs
- Sort properties alphabetically
- Unify tiles and hierarchy examples
- Wording
- Cosmetic cleanups

Extract compositing.txt from xcf.txt

Update parasites.txt:
- Replace SVN->Git
- Update contact e-mail address
- Add table of contents

Cherry-picked from commit a83791be198a7e2fd45bc38ddf566283f8a7ab78