Introduce "persistent" and "immutable" cookies

[Stefan Kanthak]

Hi @ll,

Opera should introduce a flag/setting in its cookie manager
to allow "persistent" and "immutable" cookies:

- "persistent"/"immutable" cookies are never deleted, even
when the user requests to clear ALL cookies;

- "persistent" cookies may be changed/written to by the
creating site;

- "immutable" cookies may not be changed/written to.

Rationale: retain user settings stored by websites like
<http://www.networkadvertising.org/managing/opt_out.asp> or
<http://support.microsoft.com/common/international.aspx>
but still allow to clear all (other) cookies.

Stefan