DisplaySettings: Move screen number show/hide logic into widget

This is functionally the same as before, as selecting a tab fires Show
and HideEvents. But this way, we don't need to directly access the
TabWidget, which will make using SettingsWindow simpler.

Userland/Applications/DisplaySettings/MonitorSettingsWidget.h

@@ -28,6 +28,10 @@ public:
     void apply_settings();
     void show_screen_numbers(bool);
 
+protected:
+    void show_event(GUI::ShowEvent& event) override;
+    void hide_event(GUI::HideEvent& event) override;
+
 private:
     MonitorSettingsWidget();