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>

include/libcamera/object.h

@@ -10,13 +10,14 @@
 #include <list>
 #include <memory>
 
+#include <libcamera/bound_method.h>
+
 namespace libcamera {
 
 class Message;
 template<typename... Args>
 class Signal;
 class SignalBase;
-class SlotBase;
 class Thread;
 
 class Object
@@ -36,7 +37,7 @@ protected:
 private:
 	template<typename... Args>
 	friend class Signal;
-	friend class SlotBase;
+	friend class BoundMethodBase;
 	friend class Thread;
 
 	void connect(SignalBase *signal);