StoreGate: allow parent container for DecorHandleKey

When reading/writing decorations from/on a container one requires a VarHandleKey for the container and a DecorHandleKey for the decoration, which usually leads to duplication of the container name:

SG::WriteHandleKey<MyCont> m_key{this, "Key", "MyCont"};
SG::WriteDecorHandleKey<MyCont> m_decorKey{this, "DecorKey", "MyCont.MyDecor"};

This can now be avoided by passing the "parent container" in the constructor:

SG::WriteDecorHandleKey<MyCont> m_decorKey{this, "DecorKey", m_key, "MyDecor"};

Also add testing tool and unit test to AthExStoreGateExample+ControlTest.

Implementation details:

• The DecorHandleKey classes get an extra (property) constructor that takes a VarHandleKey.
• The DecorHandleKey now holds an additional pointer to the associated parent container (nullptr by default). This is really the same as the already existing m_contHandleKey. So we could have just kept it in a (non-)owning pointer depending on the use-case. But that would require a dedicated copy constructor for this class, etc.
• In case a parent container is used in the key declaration, the pythonRepr becomes the decoration name only. Specifying the parent container in addition will result in an exception.