Move caches to ~/.cache
Move caches to ~/.cache

The system-cache (summaries and temporary repos for downloads) are moved
from ~/.local/share/flatpak/system-cache to
~/.cache/flatpak/system-cache (or wherever XDG_CACHE_HOME is) because
this is the proper location for caches.

Fixes https://github.com/flatpak/flatpak/issues/767

GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak