feat: support enforce windowed mode on start-up

This feature is sponsored by @EdgarHartel.

Issue: https://github.com/BLumia/pineapple-pictures/issues/143

app/settings.h

@@ -28,6 +28,7 @@ public:
     enum WindowSizeBehavior {
         Auto,
         Maximized,
+        Windowed,
     };
     Q_ENUM(WindowSizeBehavior)