Changes to disable cut/copy with images (fixes NB#80355).

Changes to disable cut/copy with images (fixes NB#80355).
* src/modest-text-utils.[ch], src/modest-ui-dimming-rules.c:
        * Refactored a method to check if a GtkTextBuffer has a valid
          cut/copy selection (currently it means it's not empty and does
          not contain images inside).
* src/modest-ui-actions.c:
        * Don't cut/copy if the selection is not valid.
* src/maemo/modest-msg-edit-window.c:
        * Disable default cut/copy handlers if no selection is valid.

pmo-trunk-r4155