Flatpak improvements for flathub
As noted in https://twitter.com/blandford/status/1516135165436727297 we should improve the flatpak so that it's good enough to submit to flathub. This is independent to the fixes required in #51 (closed) since this currently 'just works' with the correct app-id.
- have an extension point to support plugins
- at least one working plugin example extension
- supporting multiple output exports (eg, build the Editor and the Game from the same manifest in the way that LibreOffice does)
- "installing/managing pips" (that one needs a bit more clarification, not sure what much more than https://github.com/flatpak/flatpak-builder-tools/tree/master/pip)
- have a version that pins dependencies to exact versions for flathub
Other than that - the manifest seems to be pretty close to flathub standards already so an initial version absolutely could be posted if you fancy.