Improve app switching logic

This MR improves app-id tracking for apps which advertise a different app-id than the value we can infer from the corresponding .desktop file.

It also makes the launcher module the only component responsible of launching apps so we can more easily keep track of running apps, and ensures the previously visible application is displayed when the currently focused window is destroyed so users don't face an unexpected behavior.

Finally, it cleans up some of the code in order to get rid of compilation warnings.