Allows open flatpak manifest json projects
Currently is possible to add project definitions for meson, Autotools and others, but no one for flatpak's json manifests.
When you create a new project for App development, all works; a new json manifest is added and builder just takes it as the way to build and test.
If you have a project without json manifests and you want to add them, there is no way to do so using Builder.
I think the right way should be allow open json file, get is as a flatpak manifests and produce any required stuff to allow to create app from that file.