More efficient appstream generation
More efficient appstream generation

Rather than creating a real temp directory and
then commiting that we generate an in-memory mtree
which mostly refers to pre-existing icon file
objects, and then the newly created appdata xml.

Closes: #2753
Approved by: alexlarsson

matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak