[webrequest] Lazy-load search_state - The update_search_state function will hit the session data only if __mode is present in the form. - set_search_state keep the same semantics - Direct access to search_state in the session data is changed by an access to the new property. Closes #4875761

# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s schema"""