LibCore: Unmark timer interval being dirty after applying new interval

Previously it was never unmarked, so the timer would constantly stop
and start, which made the timer jittery especially at small intervals.

Libraries/LibCore/Timer.cpp

@@ -90,6 +90,7 @@ void Timer::timer_event(TimerEvent&)
         if (m_interval_dirty) {
             stop();
             start(m_interval_ms);
+            m_interval_dirty = false;
         }
     }