Small improvement - avoid a instanceof check.

If the type is not Component then it will fail with ClassCastException providing the same information about the problem.
Simplify the check of children's type.

It can be either Component or something else. We can use #getId(Object) only if it is a Component.

Reduces the check from two instanceof's to one.
Fix broken javadoc
WICKET-5535 Intern strings generated by Wicket that have bigger chance to be duplicated
Do not call String.valueOf(String).

This will "save us" only if the value is null.
But it is not Wicket' job to decide what keys and/or values an application can store in the ISessionStore.
Javadoc fixes, Java 7 diamonds