Re #8483. Remove unnecessary include.

Not strictly related to this ticket, but I was surprised when I made a
change in ImplicitFunctionParameter.h and everything recompiled.
This turned out the be the reason.
Re #8483. Don't leak memory via Poco XML calls.
Re #8483. Don't leak memory in Poco XML calls.
Re #8483. Don't leak memory in Poco XML calls.
Re #8483. Add forward declaration after removal of include.
Re #8483. While I'm there, thoroughly clean includes.
Re #8483. Add now-required include.

After I removed its inclusion further up the chain.
Re #8483. Add forward declarations.

Previously they were being picked up via some other header that I've
now cleaned.
Merge branch 'bugfix/8483_api_xml_memory_leaks' into develop