1. Make sure we coalesce XDG_DATA_HOME as per the spec
On my machine /etc/profile.d/flatpak.sh sets
XDG_DATA_DIRS=$HOME/.local/share/flatpak/exports/share:/var/lib/flatpak/exports/share:/usr/local/share:/usr/share
but XDG_DATA_HOME is unset.
2. Update directory creation logic to find-or-create
`enable_portal`'s current behavior is to first find a writable directory and
write the portal definition to it, then fall back to creating a directory
in the first available configuration directory.
This is incorrect in the case where one of the locations in
XDG_DATA_DIRS already has a directory, we should still prioritize
XDG_DATA_HOME.
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