Improve flatpak progress reporting
See the commit messages for the details. Essentially, this reworks the progress reporting in the
flatpak plugin to take account of the progress of runtimes required for apps.
It needs new APIs added recently in libflatpak, and another that I’ve just created an MR for: https://github.com/flatpak/flatpak/pull/3568. Without those, the existing progress reporting code is used.
Fixes: #276 (closed)