Have the hst:containercomponent by default contain the standard built in component classname for containercomponents

A hst:containercomponent is afaik always configured with:

hst:componentclassname = org.hippoecm.hst.pagecomposer.builtin.components.StandardContainerComponent

Let's make this the default value for the classname of a 'hst:containercomponent' when the property is missing.

This way, it is only needed to explicitly define it when it needs some specific custom class, which, afaik, is never needed