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++/ProxyInterfaces.h

@@ -104,7 +104,7 @@ namespace sdbus {
          * This constructor overload creates a proxy that manages its own D-Bus connection(s).
          * For more information on its behavior, consult @ref createProxy(std::string,std::string)
          */
-        ProxyInterfaces(std::string destination, std::string objectPath)
+        ProxyInterfaces(ServiceName destination, ObjectPath objectPath)
             : ProxyObjectHolder(createProxy(std::move(destination), std::move(objectPath)))
             , _Interfaces(getProxy())...
         {
@@ -119,7 +119,7 @@ namespace sdbus {
          * This constructor overload creates a proxy that manages its own D-Bus connection(s).
          * For more information on its behavior, consult @ref createProxy(std::string,std::string,sdbus::dont_run_event_loop_thread_t)
          */
-        ProxyInterfaces(std::string destination, std::string objectPath, dont_run_event_loop_thread_t)
+        ProxyInterfaces(ServiceName destination, ObjectPath objectPath, dont_run_event_loop_thread_t)
             : ProxyObjectHolder(createProxy(std::move(destination), std::move(objectPath), dont_run_event_loop_thread))
             , _Interfaces(getProxy())...
         {
@@ -135,7 +135,7 @@ namespace sdbus {
          * The proxy created this way just references a D-Bus connection owned and managed by the user.
          * For more information on its behavior, consult @ref createProxy(IConnection&,std::string,std::string)
          */
-        ProxyInterfaces(IConnection& connection, std::string destination, std::string objectPath)
+        ProxyInterfaces(IConnection& connection, ServiceName destination, ObjectPath objectPath)
             : ProxyObjectHolder(createProxy(connection, std::move(destination), std::move(objectPath)))
             , _Interfaces(getProxy())...
         {
@@ -151,7 +151,7 @@ namespace sdbus {
          * The proxy created this way becomes an owner of the connection.
          * For more information on its behavior, consult @ref createProxy(std::unique_ptr<sdbus::IConnection>&&,std::string,std::string)
          */
-        ProxyInterfaces(std::unique_ptr<sdbus::IConnection>&& connection, std::string destination, std::string objectPath)
+        ProxyInterfaces(std::unique_ptr<sdbus::IConnection>&& connection, ServiceName destination, ObjectPath objectPath)
             : ProxyObjectHolder(createProxy(std::move(connection), std::move(destination), std::move(objectPath)))
             , _Interfaces(getProxy())...
         {
@@ -167,7 +167,7 @@ namespace sdbus {
          * The proxy created this way becomes an owner of the connection.
          * For more information on its behavior, consult @ref createProxy(std::unique_ptr<sdbus::IConnection>&&,std::string,std::string,sdbus::dont_run_event_loop_thread_t)
          */
-        ProxyInterfaces(std::unique_ptr<sdbus::IConnection>&& connection, std::string destination, std::string objectPath, dont_run_event_loop_thread_t)
+        ProxyInterfaces(std::unique_ptr<sdbus::IConnection>&& connection, ServiceName destination, ObjectPath objectPath, dont_run_event_loop_thread_t)
                 : ProxyObjectHolder(createProxy(std::move(connection), std::move(destination), std::move(objectPath), dont_run_event_loop_thread))
                 , _Interfaces(getProxy())...
         {