Initial commit.
spec: Remove requirements of Track Header Box.
spec: Move mention of file type branding.
spec: Remove mentions of sample_is_non_sync_sample within boxes related to track fragment.
spec: Move mention of presence of pre-roll indicator.

Also remove redundant descriptions.
spec: Move recommendations of timescale into 4.6 Trimming of Actual Duration.
spec: Remove 4.8.5 Sample Table Box.
spec: Fix an example of ChannelMapping.
spec: Remove 4.8.1 Edit Box.
spec: Mark 4.9 Example of Encapsulation informative.
spec: Add 4.2.1 General to describe the summary.

Remove 4.8.2 Handler Reference Box and 4.8.3 Sound Media Header Box.
spec: Update normative references.