pressure-vessel: Update Flatpak-derived code to Flatpak 1.15.8

• pressure-vessel: Update Flatpak-derived code to Flatpak 1.15.8

  Many of the changes are #if 0. Main changes that have a practical effect on steam-runtime-tools:

  • Some former g_debug() has been promoted to g_info().

  • The majority of flatpak-run.c is now #if 0. The parts we use have mostly moved to flatpak-run-*.c.

  • Backports from newer GLib are now in their own translation unit. Some of them are #if 0 because they are redundant with a copy in <steam-runtime-tools/glib-backports-internal.h>.