triage: AppImage doesnt work on a clean installation

[piggz]

Describe the bug
When running the appimage, the default storage location is the read-only appimage mount. Clicking the button does nothing and clicking the create a new vm shows a dialog with a busy spinner, but does not show the list of OSs. I was able to work around this issue by creating the file:
$ cat ~/.local/share/com.example.quickgui/shared_preferences.json
{"flutter.workingDirectory":"/home/piggz/VMs"}

I also observed an unhandled exception the details of which are in the screenshot. After adding the config file, the exception does not occur.

Expected behavior
Appimage should work as well as other released packages.

Desktop (please complete the following information):

• Quickgui Versdion: 1.2.10
• OS: OpenSuse Tumbelweed
• Version : Current

Screenshots

[flexiondotorg]

I discussed this with @piggz yesterday. I think this issue might be specific to running the AppImage on a KDE Plasma desktop. But I need to find time to test that.

I am not able to reproduce the issue on GNOME or Pantheon.

That said, #146 is ultimately going to "fix" this issue.