xdg-desktop-portal-gnome is needed for example for mime type associations.
Without it, the default gnome based shell installations couldn't detect which
mime types were provided by which applications, xdg-open and the
Default Applications tab in GNOME Control Center didn't work.
This dependency was missing from postmarketos-ui-phosh, so let's add it
and xdg-user-dirs to postmarketos-base-ui-gnome to make sure it's not repeated
in every gnome based UI.
