refactor: add strong types to public API (#414)

This introduces strong types for `std::string`-based D-Bus types. This facilitates safer, less error-prone and more expressive API.

What previously was `auto proxy = createProxy("org.sdbuscpp.concatenator", "/org/sdbuscpp/concatenator");` is now written like `auto proxy = createProxy(ServiceName{"org.sdbuscpp.concatenator"}, ObjectPath{"/org/sdbuscpp/concatenator"});`.

These types are:
  * `ObjectPath` type for the object path (the type has been around already but now is also used consistently in sdbus-c++ API for object path strings),
  * `InterfaceName` type for D-Bus interface names,
  * `BusName` (and its aliases `ServiceName` and `ConnectionName`) type for bus/service/connection names,
  * `MemberName` (and its aliases `MethodName`, `SignalName` and `PropertyName`) type for D-Bus method, signal and property names,
  * `Signature` type for the D-Bus signature (the type has been around already but now is also used consistently in sdbus-c++ API for signature strings),
  * `Error::Name` type for D-Bus error names.

include/sdbus-c++/Message.h

@@ -52,10 +52,14 @@
 namespace sdbus {
     class Variant;
     class ObjectPath;
+    class InterfaceName;
+    class MemberName;
     class Signature;
     template <typename... _ValueTypes> class Struct;
     class UnixFd;
     class MethodReply;
+    class BusName;
+    using ConnectionName = BusName;
     namespace internal {
         class ISdBus;
     }
@@ -181,11 +185,11 @@ namespace sdbus {
         explicit operator bool() const;
         void clearFlags();
 
-        std::string getInterfaceName() const;
-        std::string getMemberName() const;
-        std::string getSender() const;
-        std::string getPath() const;
-        std::string getDestination() const;
+        InterfaceName getInterfaceName() const;
+        MemberName getMemberName() const;
+        ConnectionName getSender() const;
+        ObjectPath getPath() const;
+        ConnectionName getDestination() const;
         void peekType(std::string& type, std::string& contents) const;
         bool isValid() const;
         bool isEmpty() const;
@@ -281,7 +285,7 @@ namespace sdbus {
 
     public:
         Signal() = default;
-        void setDestination(const std::string& destination);
+        void setDestination(const ConnectionName& destination);
         void send() const;
     };