Add support for ObjectManager and other standard D-Bus interfaces (#55)

Fixes #50

src/Proxy.h

@@ -27,6 +27,7 @@
 #define SDBUS_CXX_INTERNAL_PROXY_H_
 
 #include <sdbus-c++/IProxy.h>
+#include "IConnection.h"
 #include <systemd/sd-bus.h>
 #include <string>
 #include <memory>
@@ -34,11 +35,6 @@
 #include <unordered_map>
 #include <mutex>
 
-// Forward declarations
-namespace sdbus { namespace internal {
-    class IConnection;
-}}
-
 namespace sdbus {
 namespace internal {
 
@@ -83,7 +79,7 @@ namespace internal {
             struct SignalData
             {
                 signal_handler callback_;
-                std::unique_ptr<sd_bus_slot, std::function<void(sd_bus_slot*)>> slot_;
+                SlotPtr slot_;
             };
             std::map<SignalName, SignalData> signals_;
         };