Added implementation of "Find in page" in editor. This, with other

Added implementation of "Find in page" in editor. This, with other
previous fixes, fixes NB#56983.
* src/widgets/modest-msg-edit-window.h:
        * New function ..._toggle_find_toolbar, to show/hide the
          toolbar.
* src/widgets/modest-msg-edit-window-ui.h:
        * Added default callback for changing find toolbar
          visibility.
* src/modest-ui-actions.[ch]:
        * Added action ..._on_toggle_find_in_page, for responding
          to find in page toggle, and showing or hiding the find
          toolbar in the editor.
* src/maemo/modest-msg-edit-window.c:
        * Added find toolbar support with its callbacks. This
          enables the user to search strings in the text. Also
          made the scroll update properly when the found selection
          changes.
* src/maemo/modest-msg-edit-window-ui.xml:
        * Added separator between "remove attachments" action and
          the attachment insertion actions.

pmo-trunk-r2112