Don't overwrite onSuccess function if passed in
Move compose attachment list code to separate class

Trying to break some stuff out of the main class, since it is getting a
bit crowded/confusing in there.
[mms] Add upload progress bar in dynamic compose screen when adding attachments.

Obviously, this only works with newer browsers that support newer
HTML5-ish features
[mms] Allow multiple independent attachment actions on the dynamic compose screen.

No need to block attachment actions (either uploading/deleting/viewing),
since any of these actions are not blocking on the PHP server/session.