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/stresstests/celsius-thermometer-proxy.h

@@ -33,6 +33,10 @@ protected:
 
     ~thermometer_proxy() = default;
 
+    void registerProxy()
+    {
+    }
+
 public:
     uint32_t getCurrentTemperature()
     {

tests/stresstests/concatenator-proxy.h

@@ -23,7 +23,6 @@ protected:
     concatenator_proxy(sdbus::IProxy& proxy)
         : proxy_(&proxy)
     {
-        proxy_->uponSignal("concatenatedSignal").onInterface(INTERFACE_NAME).call([this](const std::string& concatenatedString){ this->onConcatenatedSignal(concatenatedString); });
     }
 
     concatenator_proxy(const concatenator_proxy&) = delete;
@@ -33,6 +32,11 @@ protected:
 
     ~concatenator_proxy() = default;
 
+    void registerProxy()
+    {
+        proxy_->uponSignal("concatenatedSignal").onInterface(INTERFACE_NAME).call([this](const std::string& concatenatedString){ this->onConcatenatedSignal(concatenatedString); });
+    }
+
     virtual void onConcatenatedSignal(const std::string& concatenatedString) = 0;
 
     virtual void onConcatenateReply(const std::string& result, const sdbus::Error* error) = 0;

tests/stresstests/fahrenheit-thermometer-proxy.h

@@ -33,6 +33,10 @@ protected:
 
     ~thermometer_proxy() = default;
 
+    void registerProxy()
+    {
+    }
+
 public:
     uint32_t getCurrentTemperature()
     {
@@ -71,6 +75,10 @@ protected:
 
     ~factory_proxy() = default;
 
+    void registerProxy()
+    {
+    }
+
 public:
     sdbus::ObjectPath createDelegateObject()
     {