Bug 995605 - [Sora][Message][MMS]Can't add the text in content when forw...

...ard 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.