Work to make attachments be opened in a single instance (raise window

Work to make attachments be opened in a single instance (raise window
if they're already openened) (fixes NB#65279).
* src/widgets/modest-window-mgr.[ch]:
        * Now modest_window_mgr_register_header accepts an alt_uid
          parameter in case the header does not provide an uid.
* src/maemo/modest-msg-view-window.c:
        * Now we pass a hash base to the create_temp_stream method used
          to create a temporary file to view an attachment. This hash
          is used to open always the file in the same directory.
        * Now the attachment messages are opened passing a "attachment
          id" that works similar to a message id. This identifies the
          message in the window manager properly.
* src/modest-ui-actions.c, src/dbus_api/modest-dbus-callbacs.c:
        * Use new modest_window_mgr_register_header call.
* src/maemo/modest-maemo-utils.[ch]:
        * Now ..._create_temp_stream accepts an additional parameter
          hash_base. This hash_base is used to generate the "random"
          directory the temporary file is created. This is used to
          force files created from attachments get the same path.

pmo-trunk-r3529