WICKET-5444 Make org.apache.wicket.pageStore.memory.PageTable public so custom impls of IDataStoreEvictionStrategy can be made

(cherry picked from commit ded718cf9ce1d8a61e7ff33327e29c22ddce4681)
Add 'change' Ajax link next to the captcha image to show how to refresh/change it.

(cherry picked from commit e594bb85675eab0a3dfd7fdec738fada07b9607b)