Make IMAP_EMAIL_CONTENT support push

I was looking at the code for imap_email_content and noticed it doesn’t support push, whereas the imap component supports both push and pull.

I was thinking that the imap_email_content component could simply subclass the imap component to support push (and reduce code duplication).

Is there any particular reason why it can’t be done this way? (i.e. coding standards, something about subclassing an existing component being more difficult than it seems, etc)

If not, I might see if I can implement it (unless someone else wants to)