Bug 995605 - [Sora][Message][MMS]Can't add the text in content when forward a media MMS r=schung

The issue is that in compose.js, "pointer-events: none" is added after it's
removed, in this case.

This patch ensures that SMIL.parse always calls the callback in an asynchronous way,
so that the caller does not have any surprise.
Merge branch 'v1.3' into v1.3t