Memory improvements in the extension system

Some improvements to memory management are made in the extension system.

• The extension objects, which used to be carelessly allocated on the heap and left there to leak are now owned by the "Extension database".
• A memory issue with the ImageMagick extension is fixed: the memory was allocated with array new but freed with non-array delete. Previously this issue was never seen because the destructor was not called, now it is not seen because we use std::vector instead of calling new and delete manually.

Note: The extension system still leaks massively, but somewhat less massively than before.