Make it easier to override parts of SystemMapper

https://github.com/wicketstuff/core/pull/476 has a use case where SystemMapper needs to be partially modified.
Currently the only way is to copy/paste the whole class and do the customizations.

It would be better if there are factory methods for all the system mappers. This way a specialization, like PortletSystemMapper, can provide custom mappers only for the ones which require special behavior in Portlet environment.