refactor: improve Object vtable registration (#388)

This improves the D-Bus object API registration/unregistration by making it more flexible, more dynamic, closer to sd-bus API design but still on high abstraction level, and -- most importantly -- less error-prone since no `finishRegistration()` call is needed anymore.

CMakeLists.txt

@@ -89,6 +89,8 @@ set(SDBUSCPP_HDR_SRCS
 set(SDBUSCPP_PUBLIC_HDRS
     ${SDBUSCPP_INCLUDE_DIR}/ConvenienceApiClasses.h
     ${SDBUSCPP_INCLUDE_DIR}/ConvenienceApiClasses.inl
+    ${SDBUSCPP_INCLUDE_DIR}/VTableItems.h
+    ${SDBUSCPP_INCLUDE_DIR}/VTableItems.inl
     ${SDBUSCPP_INCLUDE_DIR}/Error.h
     ${SDBUSCPP_INCLUDE_DIR}/IConnection.h
     ${SDBUSCPP_INCLUDE_DIR}/AdaptorInterfaces.h