Draft: packaging/flatpak: add manifest for GNOME Builder integration

This just imports the Flatpak manifest of Inkscape from Flathub, into packaging/flatpak/ where various other packaging bits seem to be stored. Happy to iterate on it with your direction.

It's less concerned with being "the canonical flatpak manifest" and more about describing all the dependencies for Inkscape so that you can just clone the repository with GNOME Builder, then click Run. Builder will automatically locate Flatpak manifests in projects and setup the build container/SDK so that even immutable operating systems (Silverblue, etc), can build/run without modifying the host.

With this in place, you really just install Builder, clone the repository, click Run.

After it builds, you also have Clang integration for diagnostics, code completion, ability to apply .clang-format on save, and a lot of other useful features for C/C++ projects including gdb, valgrind, and profiling integration.

Granted, if this isn't something you don't want, I totally understand. I just needed to test some C++ features last night and figured Inkscape was a nice sizeable project to do so.

P.S. I need more C++ users of Builder so that I can improve our support, particularly with auto-indenters :)