efl_access_object: Support org.a11y.atspi.Value.Text property

efl_access_object: Support org.a11y.atspi.Value.Text property

The application may set the "value_format" attribute to one of the
following values in order to customize what is read by the Screen Reader:

    1. "percent" (the default) - current value normalized as a percentage
       of the range [minimum value, maximum value],
    2. "number" - current value verbatim
    3. "text" - customized textual representation is used instead of current value

Option "text" must be acompanied by registration of a callback that would provide
relevant textual representation of current value:

    efl_access_object_value_text_cb_set(slider, value_text_cb, NULL);
    efl_access_object_attribute_append(slider, "value_format", "text");

    static char *value_text_cb(void *data, Evas_Object *obj)
    {
        return strdup("This information is set by value text callback");
    }

@tizen_only

Change-Id: Ic0d544dc163dff20551dddbcfe92ffcbd1876541