elm: restore old elm_web behaviour

Everything that is owned must have a free function. In commit
1afd3c215f the string was
converted to const(string) it was not const before.
So this is converting back to the old behaviour.

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D7626

src/lib/elementary/elm_web.eo

@@ -278,8 +278,7 @@ abstract Elm.Web extends Efl.Ui.Widget implements Efl.Ui.Legacy, Efl.Ui.Zoom
               The string returned must be freed by the user when it's done
               with it.
             ]]
-            /* FIXME: owned const string? */
-            return: string @owned @warn_unused; [[A newly allocated string, or
+            return: mstring @owned @warn_unused; [[A newly allocated string, or
                                                       $null if nothing is selected
                                                       or an error occurred.]]
          }

src/modules/elementary/web/none/elm_web_none.c

@@ -147,7 +147,7 @@ _elm_web_none_elm_web_bg_color_get(const Eo *obj EINA_UNUSED, Elm_Web_None_Data
    if (a) *a = 0;
 }
 
-EOLIAN static const char*
+EOLIAN static char*
 _elm_web_none_elm_web_selection_get(const Eo *obj EINA_UNUSED, Elm_Web_None_Data *_pd EINA_UNUSED)
 {
    return NULL;