Work to fix glitches with progress toolbar (fixes NB#78458).

Work to fix glitches with progress toolbar (fixes NB#78458).
* src/maemo/modest-progress-bar-widget.c:
        * Now we make sure we never set a NULL string as label of the
          progress bar, and we get an initial " " string. This is for
          getting always the expected height. This should prevent some
          toolbar resizements.
* src/maemo/modest-msg-view-window.c,
  src/maemo/modest-main-window.c:
        * Rework the toolbar buttons size management. This way the
          progress toolbar does not cause a resize of the other buttons.
* src/maemo/ui/modest-msg-view-window-ui.xml,
  src/maemo/ui/modest-main-window-ui.xml:
        * Add position attributes to toolbar.

pmo-trunk-r3910