Create flatpak manifest

[hanaral]

This is not only necessary for the new packaging method for elementary OS, but will have some incredibly powerful advantages beside that:

• Users can literally compile and test the app with a single dependency (io.elementary.Sdk)
• You can create git release artefacts in the form of the .flatpak bundle
• Users can install and uninstall the app with virtually no repercussions This does require some research into how disk image reading/writing can be done from the sandbox, but I have no doubt that it's possible. It's also better to handle this early so all the researching/porting isn't left until after the app is release-worthy.