Multiple attachments can be select at once

There is a problem with attachment labels in dialog. From some reason
it takes some delay for Lightning to set the label. These labels
must be reset by our addon because Lightning shows them uri encoded
which is ugly. But if our addon does it before Lightning, Lightning
then resets them back to ugly ones.
Now it is solved using timeout which probably isn't the best solution.
IT requires longer timeout when there is more items.
I spent quite a lot of time trying to figure out some better
solution using MutationObserver but without any luck.

Also nsIFilePicker dialog returns nsISimpleEnumerator which is also
somehow broken. It ends in infite loop listing only first item.
That's why there is this weird thing:
  var files = [f for (f in fixIterator(fp.files,
    Components.interfaces.nsILocalFile))];
which converts it to array and casts to nsILocaFile.
Changing options in attachments popupmenu are not visible

Using different event UI can be updated without
noticing by user.
Fixed items in popumenus for attachments

Listed options differ depending on type of attachemnt (eee, url, local file).
Fixed opening non 3e attachments