Read the 'altt' item property

[dukesook]

See ISO/IEC 23008-12 Section 6.5.21:

aligned(8) class AccessibilityTextProperty
extends ItemFullProperty('altt', version = 0, flags = 0) {
  utf8string alt_text;
  utf8string alt_lang;
}

[changeset-bot]

🦋 Changeset detected

Latest commit: ccbdeb6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
mp4box	Minor

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[DenizUgur]

@dukesook can you add a minor changeset?