Make the state-cookie optional

DHTML Menu has always used a cookie to remember which menus are open. This was required before the active trail was automatically expanded. Now, there are many cases where you might not want it, particularly when you also want to limit the structure to one open menu at a time.

Therefore, this change will:

1.) Add a new setting to enable or disable the cookie
2.) Turn it off when "keep only one menu open" is enabled.

The changeset is already in #473356: DHTML Menu takes another leap, and will need to be isolated to a separate patch for this issue.