Fix util::timer: add timer duration to tracked time point instead of replacing it with now()

libs/util/include/psemek/util/clock.hpp

@@ -42,7 +42,7 @@ namespace psemek::util
 			return duration().count();
 		}
 
-	private:
+	protected:
 		time_point_type start_;
 	};
 

libs/util/include/psemek/util/timer.hpp

@@ -17,7 +17,7 @@ namespace psemek::util
 		{
 			if (this->duration() >= duration_)
 			{
-				this->restart();
+				this->start_ += std::chrono::duration_cast<typename Clock::duration>(duration_);
 				return true;
 			}