Add CMessage::Clone()

Due to (intentional) lack of CFooMessage::operator=(CMessage), it was
a bit clumsy to do such copy-conversions:

    CMessage Copy = Message;
    CJoinMessage& JoinMsg = static_cast<CJoinMessage&>(Copy);
    // ...

vs.

    CJoinMessge JoinMsg;
    JoinMsg.Clone(Message);
    // ...

Alternatively, copy ctor(CMessage) and assignment operator=(CMessage)
could have been added to all CMessage subclasses. I've been trying to
avoid that, to make these operations very explicit. It has helped a
lot so far by preventing accidental copies.

include/znc/Message.h

@@ -31,6 +31,8 @@ public:
 	explicit CMessage(const CString& sMessage = "");
 	CMessage(const CNick& Nick, const CString& sCommand, const VCString& vsParams = VCString(), const MCString& mssTags = MCString::EmptyMap);
 
+	void Clone(const CMessage& Other);
+
 	// ZNC <-> IRC
 	CIRCNetwork* GetNetwork() const { return m_pNetwork; }
 	void SetNetwork(CIRCNetwork* pNetwork) { m_pNetwork = pNetwork; }

src/Message.cpp

@@ -29,6 +29,13 @@ CMessage::CMessage(const CNick& Nick, const CString& sCommand, const VCString& v
 	InitTime();
 }
 
+void CMessage::Clone(const CMessage& Message)
+{
+	if (&Message != this) {
+		*this = Message;
+	}
+}
+
 CString CMessage::GetParams(unsigned int uIdx, unsigned int uLen) const
 {
 	VCString vsParams;