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.
Re #8483. Add now-required include.
Re #8483. Add a now-required include.
Re #8483. Correct the spelling of the filename.
Re #8483. Use Poco::AutoPtr to avoid leaking memory.
Merge remote-tracking branch 'origin/bugfix/8483_api_xml_memory_leaks'