refactor: improve Proxy signal subscription (#389)

This makes D-Bus proxy signal registration more flexible, more dynamic, and less error-prone since no `finishRegistration()` call is needed. A proxy can register to a signal at any time during its lifetime, and can unregister freely by simply destroying the associated slot.

tests/perftests/perftests-proxy.h

@@ -22,7 +22,6 @@ protected:
     perftests_proxy(sdbus::IProxy& proxy)
         : proxy_(&proxy)
     {
-        proxy_->uponSignal("dataSignal").onInterface(INTERFACE_NAME).call([this](const std::string& data){ this->onDataSignal(data); });
     }
 
     perftests_proxy(const perftests_proxy&) = delete;
@@ -32,6 +31,11 @@ protected:
 
     ~perftests_proxy() = default;
 
+    void registerProxy()
+    {
+        proxy_->uponSignal("dataSignal").onInterface(INTERFACE_NAME).call([this](const std::string& data){ this->onDataSignal(data); });
+    }
+
     virtual void onDataSignal(const std::string& data) = 0;
 
 public: