knsrc for AppImage

If we put

```
[KNewStuff3]
Name=AppImage
Categories=233
InstallPath=bin

ProvidersUrl=https://www.linux-appimages.org/ocs/providers.xml
InstallationCommand=chmod a+x %f
```

into `/etc/xdg/org.linux-appimages.knsrc`, then on `neon-useredition-20170504-1018-amd64.iso` we get in Discover: Software Addone -> AppImage -> ...

Open issues
- we possibly want fancier tooling for installing --> TODO for AppImageKit. Deliver a library or CLI/GUI tool, possibly in the `appimaged` package
- gpg signing is not taken into account --> Could be handled by the above
- ocs lists everything, not just appimages --> TODO for the Discover client? Or for the OCS server?
- see last point, needs to have all categories listed --> `Categories=233` seems to do that
- AppImages should show up under "Applications" rather than "Application addons", and be sorted into the usual categories. Maybe if the same application is available in multiple formats (distribution, AppImage stable, AppImage nightly, Flatpak, Snap) then the different installation sources and "channels" (stable, nightly,...) should be selectable from the "product detail page".