Add custom prefix to user generated desktop files made from shortcuts KCM

The shortcuts KCM re-uses desktop files to store launchers for custom commands. This is extremely sensible design as it is something being launched and the design we want. Semantically we're creating a new entry point. It's the only way you'll get the right cgroup mapped on launch.

However the name entered by the user could clash with something on the system which is something we don't want. Most apps are prefixed with the correct reverse DNS prefix so it's a rare case, but not all.

This was seen in the wild by a user overriding alacritty.desktop by accident.

Adding a custom prefix for user-generated files should avoid any potential clash.

Fixed-in: 6.2 BUG: 492754