Launch does not select the app having a .desktop file

[Michał Sawicz]

Steps:
- install Multipass
- click the Launch button

Expected:
- the Multipass tray icon is run

Current:
- the multipass CLI is run and exits

I modified the snap to correctly report a .desktop file:

```
…
    "apps": [
      {
        "snap": "multipass",
        "name": "gui",
        "desktop-file": "/var/lib/snapd/desktop/applications/multipass_gui.desktop"
      },
…
```

But that doesn't help.

Incidentally, if you use the implicit handling (a snap/gui/<app>.desktop file), that will only show up in the API if it exactly matches the app name…

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: gnome-software 3.30.6-2ubuntu4.19.04.1
ProcVersionSignature: Ubuntu 5.0.0-29.31-generic 5.0.21
Uname: Linux 5.0.0-29-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.10-0ubuntu27.1
Architecture: amd64
CurrentDesktop: GNOME
Date: Fri Sep 20 14:28:25 2019
InstalledPlugins:
 gnome-software-plugin-flatpak N/A
 gnome-software-plugin-snap 3.30.6-2ubuntu4.19.04.1
SourcePackage: gnome-software
UpgradeStatus: Upgraded to disco on 2019-04-09 (163 days ago)