Add JSON manifest

[fsobolev]

While yml manifests are supported by flatpak-builder, they seem to be unsupported by GNOME Builder unfortunately. I added json manifest that can be used there. It's identical to yml manifest, except for the sources: instead of building from specific commit in repository, it builds the app from current directory, allowing to test changes in one click. I didn't touch yml file, only added json. I hope it will be useful, if not for you, then for other contributors who use GNOME Builder.

[fabrialberio]

Ok, I'll also add .dev to the end of the manifests (like Bottles does), to make it clear that they are not the release manifest