Implement "search messages" action (fixes NB#57209).

* src/modest-platform.h:
        * New method to invoke the search messages dialog
* src/gnome/modest-platform.c:
        * Stub implementation for ...show_search_messages
* src/widgets/modest_msg_edit_window-ui.h:
        * Added callback for search messages action
* src/modest-ui-actions.[ch]:
        * Added new ui action to show the search messages dialog,
          relying on modest platform implementation.
* src/maemo/modest-main-window-ui.h:
        * Added callback for search messages action
* src/maemo/modest-platform.c:
        * New ..._show_search_messages method. Shows the osso
          global search dialog for searching emails.
* src/maemo/ui/modest-msg-view-window-ui.xml:
        * Fix and show search messages action.

Other issues:
* libmodest-dbus-client/libmodest-dbus-client-1.0.pc.in:
        * Fixed -l declaration.

pmo-trunk-r2071