Refactoring sort dialog code to be able to implement easier a gnome

Refactoring sort dialog code to be able to implement easier a gnome
counterpart
* src/modest-platform.h:
        * Now only sort dialog creation is done in platform code.
* Added src/widgets/modest-sort-criterium-view.[ch]:
* src/modest-ui-actions.c
* src/maemo/modest-platform.c:
        * Implementation of method to create hildon platform sort
          dialog.
* Added src/maemo/modest-hildon-sort-dialog.[ch]:
* src/modest-utils.[ch]:
        * Added code to change the sorting of the header view using
          information from a ModestSortCriteriumView dialog.

pmo-trunk-r4706