refactor: let callbacks take message objects by value (#367)

Signatures of callbacks async_reply_handler, signal_handler, message_handler and property_set_callback were modified to take input message objects by value, as opposed to non-const ref.

The callee assumes ownership of the message. This API is more idiomatic, more expressive, cleaner and safer. Move semantics is used to pass messages to the callback handlers. In some cases, this also improves performance.

include/sdbus-c++/ConvenienceApiClasses.inl

@@ -317,7 +317,7 @@ namespace sdbus {
         if (propertySignature_.empty())
             propertySignature_ = signature_of_function_input_arguments<_Function>::str();
 
-        setter_ = [callback = std::forward<_Function>(callback)](PropertySetCall& call)
+        setter_ = [callback = std::forward<_Function>(callback)](PropertySetCall call)
         {
             // Default-construct property value
             using property_type = function_argument_t<_Function, 0>;
@@ -580,7 +580,7 @@ namespace sdbus {
     {
         assert(method_.isValid()); // onInterface() must be placed/called prior to this function
 
-        auto asyncReplyHandler = [callback = std::forward<_Function>(callback)](MethodReply& reply, const Error* error)
+        auto asyncReplyHandler = [callback = std::forward<_Function>(callback)](MethodReply reply, const Error* error)
         {
             // Create a tuple of callback input arguments' types, which will be used
             // as a storage for the argument values deserialized from the message.
@@ -656,7 +656,7 @@ namespace sdbus {
 
         proxy_.registerSignalHandler( interfaceName_
                                     , signalName_
-                                    , [callback = std::forward<_Function>(callback)](Signal& signal)
+                                    , [callback = std::forward<_Function>(callback)](Signal signal)
         {
             // Create a tuple of callback input arguments' types, which will be used
             // as a storage for the argument values deserialized from the signal message.

include/sdbus-c++/IConnection.h

@@ -286,7 +286,7 @@ namespace sdbus {
          * The syntax of the match rule expression passed in match is described in the D-Bus specification.
          * The specified handler function callback is called for each incoming message matching the specified
          * expression. The match is installed synchronously when connected to a bus broker, i.e. the call
-         * sends a control message requested the match to be added to the broker and waits until the broker
+         * sends a control message requesting the match to be added to the broker and waits until the broker
          * confirms the match has been installed successfully.
          *
          * Simply let go of the slot instance to uninstall the match rule from the bus connection. The slot

include/sdbus-c++/TypeTraits.h

@@ -63,10 +63,10 @@ namespace sdbus {
 
     // Callbacks from sdbus-c++
     using method_callback = std::function<void(MethodCall msg)>;
-    using async_reply_handler = std::function<void(MethodReply& reply, const Error* error)>;
-    using signal_handler = std::function<void(Signal& signal)>;
-    using message_handler = std::function<void(Message& msg)>;
-    using property_set_callback = std::function<void(PropertySetCall& msg)>;
+    using async_reply_handler = std::function<void(MethodReply reply, const Error* error)>;
+    using signal_handler = std::function<void(Signal signal)>;
+    using message_handler = std::function<void(Message msg)>;
+    using property_set_callback = std::function<void(PropertySetCall msg)>;
     using property_get_callback = std::function<void(PropertyGetReply& reply)>;
 
     // Type-erased RAII-style handle to callbacks/subscriptions registered to sdbus-c++