UnifiedEmail: Support Email load more content function.

 - If the mail's content didn't download complete, provider one action button
   to make the user could load more content.
 - If the mail's loaded status is fetching, it will display one progress bar.

Change-Id: I21dd994e9bbfb380d7590cec2a32732098f45efa
UnifiedEmail: Display the attach file menu.

 - Display the attach file menu if the user enable "Add any file as attachment"
   item. And after the user press this menu, it will let the user choose any
   file as the attachment for the compose message.

Change-Id: Iead5c6695af91acca0c17ff6f6bbc69d2d123a52
UnifiedEmail: Display the pick recipients button.

 - Display the pick recipients button if the user enable "Select recipients
   from Contacts" item. And after the user press this button, it will go to
   the Contacts list to let the user pick the recipients as to/cc/bcc list.

Change-Id: I3221afc97aed16753596dfcd45526709756a540e