requested to merge Tamaranch/ristretto:flatpak-add-tumbler-dependency into master
This makes the Flatpak application self-contained from a thumbnail
perspective, as it was already from a configuration perspective. So
there is no need to worry about whether we are in the Flatpak context
when building the thumbnail path: the cache directory is always
$XDG_CACHE_HOME, which varies depending on the context.
Related to xfce/tumbler!21 (merged).