Make product, language and session optional URL parts

Rationale

Every setup includes a default product and a default language which can be used when no value has been passed through the URL.

Many applications do not have more than one product or more than one language, so always passing it is overhead. In case non-default values are chosen, they'll again be passed through the known URL format.

Parsing product, language and session information will no longer be done through Apache rewrite rules but within the scriptlet API itself. A regular expression is used to parse the string and this expression needs to be flexible enough to detect omitted values.

Apache setup will be simpler as the required rewrite rule is much simpler.

n/a

Should have no impact.

n/a

- http://xp-framework.net/rfc/contrib/rfc0157.diff Implementing patch

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