Use a demo app for all examples and tutorials

[allanday]

As mentioned in #74, the apps that are used to demo Flatpak aren't ideal and we could do with a demo app of our own, in order to give us some control and allow us to show off Flatpak's features.

Some things the app should probably do:

• Use GTK or Qt
• Bundle 2/3 modules
• Use recommended/common sandbox permissions
• Some cleanup
• Rename some files?
• Make use of portals?
• Look nice when it's run

[grulja]

I'll try to come up with something next week if you want a Qt demo app.

[allanday]

I've done a quick mockup for the app here:

https://raw.githubusercontent.com/flatpak-design-team/flatpak-website-mockups/master/demo-app.png

[allanday]

@grulja has created the demo app at https://github.com/grulja/flatpak-demo/ . We ought to figure out what's left to do before we can start using it in the docs.

[allanday]

The demo app is now hosted on Flathub, but it needs some extra modules adding to the manifest.