[snap] chromium cannot launch programs from links like apt-url

[Michel-Ekimia]

To reproduce

- Install 18.04
- Install chromium snap
- Click on a apturl: link
- choose open with xdg-open

Result :

- Nothing happens

Expected result :

- the app install window should be launched

If launching programs is not possible on snap version at least we should warn the user.

[Olivier Tilloy (osomon)]

Corresponding log on stderr:

    user-open error: Supplied URL scheme "apt" is not allowed

[Olivier Tilloy (osomon)]

See https://github.com/snapcore/snapd/blob/master/usersession/userd/launcher.go#L60 :

    allowedURLSchemes = []string{"http", "https", "mailto", "snap", "help"}