Add Flatpak manifest to enable one-click building in Builder
For new contributors, it would be great to have a streamlined way to build Inkscape from git. GNOME Builder enables such a workflow: You just clone the repository and click the build button, everything else is automatic.
As someone who does lots of smaller design contributions to third-party apps, this is invaluable for me. It abstracts away all the stuff I don't care about and just lets me quickly test an app/branch to give feedback on it.
To enable this workflow in Inkscape, all that's needed is adding a Flatpak manifest in the root folder of the repository.
This is a manifest that Jordan Petridis and Chris Davis made a while back, and which I'm personally using to build Inkscape: org.inkscape.Inkscape.json