Invoke desktop2app automatically when pkg installed a new desktop file

Invoke desktop2app automatically when pkg installed a new desktop file
* Put a QFilesystemWatcher on `/usr/local/share/applications`
* When a new desktop file arrives, run `desktop2app` on it

Before we can do this we should change `desktop2app` so that it creates wrapper apps like these ones:

https://github.com/helloSystem/Utilities/tree/master/3D%20Printing

Namely, once the user has uninstalled the application via e.g., `pkg`, the wrapper app should not just be broken but instead should ask the user to reinstall the package which the desktop file originally belonged to.