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/Applications/SystemMonitor/ProcessMemoryMapWidget.cpp

@@ -110,6 +110,7 @@ ProcessMemoryMapWidget::ProcessMemoryMapWidget()
 
     m_table_view->set_key_column_and_sort_order(0, GUI::SortOrder::Ascending);
     m_timer = add<Core::Timer>(1000, [this] { refresh(); });
+    m_timer->start();
 }
 
 void ProcessMemoryMapWidget::set_pid(pid_t pid)