Provide wrapper for g_action_map_lookup_action

	* src/main-window.c (bw_get_action): wrapper for
	g_action_map_lookup_action;
	(bw_action_set_boolean), (bw_action_get_boolean),
	(bw_action_set_string), (bw_action_set_enabled),
	(hide_change_state), (balsa_window_new),
	(balsa_window_update_book_menus): use it;
	(bw_set_filter_menu): use g_simple_action_set_state, to avoid
	recursion;
	(update_view_menu): remove redundant call to
	balsa_message_set_wrap.