[CSWH] WindowContextConfig#isUnknownWindowIdsAllowed() should default to "true"

If we like to support <a target="xyz"...> then we need to allow managing of unknownWindowIds. The windowId is set by the window.name of the browser tab. This will also allow mixed operation mode with other frameworks which utilize the window.name for browser tab detection.

This mode is also essential for EARs and portlets!

Please note that this will not introduce a security problem as the windowId is just a sub-context of the session (which is well protected already).