libcamera: signal: Split Slot implementation to reusable classes

Move the Slot* classes to bound_method.{h,cpp} and rename them to
Bound*Method*. They will be reused to implement asynchronous method
invocation similar to cross-thread signal delivery.

This is only a move and rename, no functional changes are included.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/include/message.h

@@ -11,8 +11,8 @@
 
 namespace libcamera {
 
+class BoundMethodBase;
 class Object;
-class SlotBase;
 class Thread;
 
 class Message
@@ -44,12 +44,12 @@ private:
 class SignalMessage : public Message
 {
 public:
-	SignalMessage(SlotBase *slot, void *pack)
-		: Message(Message::SignalMessage), slot_(slot), pack_(pack)
+	SignalMessage(BoundMethodBase *method, void *pack)
+		: Message(Message::SignalMessage), method_(method), pack_(pack)
 	{
 	}
 
-	SlotBase *slot_;
+	BoundMethodBase *method_;
 	void *pack_;
 };