Fixed some reference leaks of attachments. I also moved from using

Fixed some reference leaks of attachments. I also moved from using
GList* of TnyMimeParts to use TnyLists to maintain a more intuitive list
of references.
* src/widget/modest-gtkhtml-msg-view.[ch]:
        * Use TnyList of attachments
* src/widgets/modest-msg-edit-window.h:
        * Use TnyList instead of GList for attachments.
* src/widgets/modest-msg-view.[ch]:
        * Use TnyList instead of GList for attachments.
* src/widgets/modest-attachment-view.c:
        * Now the ModestAttachmentView always keep a reference
          to the mime part with g_object_ref.
* src/widgets/modest-msg-view-window.h:
        * Use TnyList instead of GList for attachments.
* src/widgets/modest-attachments-view.[ch]:
        * Missing unreferencing on iteration.
        * Use TnyList instead of GList for attachmetns.
* src/modest-tny-msg.c:
        * Use TnyList and unrefernce an attachment list properly.
* src/modest-ui-actions.c:
        * Mail operations for saving to drafts or sending mails from
          edit window shouldn't keep a reference to the edit window.
        * Fixed minor bug assigning a g_strdup.
        * Some frees in error cases in modest_ui_actions_on_send.
* src/maemo/modest-msg-edit-window.c:
        * Fixed some leaked references.
        * Use TnyList instead of GList for lists of attachments.
* src/maemo/modest-msg-view-window.c:
        * Use TnyList instead of GList for lists of attachments.
        * Fixed some counting leaks.
* src/modest-utils.[ch]:
        * New method modest_list_index (), that finds the ordinal index
          of an item in a list.
* src/modest-ui-dimming-rules.c:
        * Use TnyList instead of GList for lists of attachments.

pmo-trunk-r3996