[Back-port to 3.2] Expose cookie methods to subclasses of LoginPanel

As mchatzidakis requests in CMS-6954, the following two methods should be made protected:

private void setCookieValue(final String cookieName, final String cookieValue, final int maxAge)
private String getCookieValue(final String cookieName)

in order to support easier extending of the login screen with features that make use of cookies. Our login screen is already made to be extensible, and these are 2 nice convenience methods which were unfortunately declared private.