Priority flags changes.

Priority flags changes.
* src/widgets/modest-gtkhtml-msg-view.c:
        * Remove all internal priority mask to delefate on headers view.
* src/widgets/modest-header-view.c:
        * Replace old priority comparison code to be proper for the
          priority flags after last changes in tinymail.
* src/widgets/modest-header-view-render.c:
        * If we try to get the pixbuf for 0, we get NULL pixbuf.
        * Update the priority flags management showing priority pixbufs.
* src/widgets/modest-mail-header-view.c:
        * Use new priority flags in tinymail.
* src/widgets/modest-mozembed-msg-view.c:
        * Use mail header view priorities instead of storing a local
          copy of priorities.
* src/maemo/modest-msg-edit-window.c:
        * Use new priority flags api from tinymail.
* src/maemo/modest-platform.c:
        * Use new priority flags api from tinymail.
* src/modest-tny-send-queue.c:
        * Use the new priority/suspended flags api. Now we don't unset
          the flags if message send failed.

Mozembed changes:
* src/widgets/modest-mozembed-msg-view.c:
        * Use scroll area (pending on changes in gtkmozembed to be
          scrollable).
* src/widgets/modest-mozembed-mime-part-view.c:
        * Implementation of ModestISearchView interface for incremental
          search.
* src/maemo/modest-msg-view-window.c:
        * More integration with mozembed (adaptation for future
          scrollable gtkmozembed widget).

pmo-trunk-r3595