Canonical location in `PREFIX` for menu item artifacts

The CEP preview of `menuinst` creates shortcuts for Linux, macOS and Windows in three default locations:

- Linux: `~/.local/share/applications`
- macOS: `~/Applications`
- Windows: whatever the default locations for Start Menu, Quick Launch and Desktop are.

For this we create full files with computable paths, so we can re-obtain these paths at uninstall time if needed. The problem is that these files might not be there by the time we want to uninstall them (Windows users might rename or move the shortcuts, and they'd still work; macOS users might put the `.app` directory somewhere else; I am not expecting Linux users to mess with their `.desktop` files but you never know).

Instead of doing this, @goanpeca and I were talking about creating the actual menu files in `PREFIX` itself (under `var/menuinst` or similar), and the symlink to this canonical location from the system parts. This needs to be tested (specially on Windows, with different letter units?), but it _should_ work. It would also have the benefit of having an easy-to-find location for the generated shortcuts without having to re-run `menuinst` to find out.