Bug 882094: display attachment file names in MMS, r=julienw

* If an MMS attachment has an thumbnail preview:
  - display it in a 80x80 px placeholder (can be extended to 120x80 or 80x120);
  - display the attachment size if in the Compose window;
  - do not display the file name.

* Otherwise (e.g. audio, video, large images):
  - display the attachment type as a 80x80 px icon (audio, video, image, other);
  - display the attachment size if in the Compose window;
  - display the file name, both in the Compose window and in the message thread.

This patch now uses two separate templates for these two use cases. We still rely on `<iframe class="attachment">` elements to work around the `contentEditable` issue in the Compose area, and a `preview` / `nopreview` class is added accordingly.

Side note: the `bubbleEvents` method has been slightly re-rewritten to be more self-explanatory, and some basic cleanup has been done on the HTML & CSS front.