XDG_CURRENT_DESKTOP can contain a list [1]. Allow users to
set a list such as XDG_CURRENT_DESKTOP=asdf:wlroots. That way, users
can use xdpw in their non-Sway compositor without specifying
XDG_CURRENT_DESKTOP=sway and without having to edit the portal
definition.
[1]: 9a2582d035/src/portal-impl.c (L189)