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.

tools/xml2cpp-codegen/AdaptorGenerator.cpp

@@ -82,7 +82,7 @@ std::string AdaptorGenerator::processInterface(Node& interface) const
     body << "class " << className << endl
             << "{" << endl
             << "public:" << endl
-            << tab << "static constexpr const char* INTERFACE_NAME = \"" << ifaceName << "\";" << endl << endl
+            << tab << "static inline const sdbus::InterfaceName INTERFACE_NAME{\"" << ifaceName << "\"};" << endl << endl
             << "protected:" << endl
             << tab << className << "(sdbus::IObject& object)" << endl
             << tab << tab << ": object_(&object)" << endl