flatpak-builder: Add ability to lint manifests

[alexlarsson]

From @TingPing on March 13, 2017 4:40

It would be useful for Gnome Builder if it could parse the manifest for warnings or errors without building the project.

Copied from original issue: flatpak/flatpak#623

[alexlarsson]

At least it now prints warnings for unknown propeties

[alexlarsson]

From @ace13 on May 3, 2017 8:12

You can skip all the build steps, and only parse the manifest, by doing something like;

flatpak-builder --download-only --disable-download directory_name com.application.manifest.json

Currently using that method myself for validation. Worth noting is that directory_name doesn't need to exist, and won't be created by the command.

[alexlarsson]

From @TingPing on May 3, 2017 9:36

I actually believe this will be the best method to lint them going forward: https://github.com/TingPing/flatpak-manifest-schema

External tools and editors will do a better job than flatpak-builder probably.