LibCore+Userland: Don't auto-start new Core::Timers

This was unintuitive, and only useful in a few cases. In the majority,
users had to immediately call `stop()`, and several who did want the
timer started would call `start()` on it immediately anyway. Case in
point: There are only two places I had to add a manual `start()`.

Userland/Services/Taskbar/ClockWidget.cpp

@@ -35,6 +35,7 @@ ClockWidget::ClockWidget()
             set_tooltip(Core::DateTime::now().to_deprecated_string("%Y-%m-%d"sv));
         }
     });
+    m_timer->start();
 
     m_calendar_window = add<GUI::Window>(window());
     m_calendar_window->set_window_type(GUI::WindowType::Popup);