Search in $XDG_DATA_DIRS for portals

[concatime]

Hi, just like how D-Bus finds services, could you search in XDG_DATA_DIRS for portals?

I recommend this logic:

• If XDG_DESKTOP_PORTAL_DIR is set, look into it (personally, XDG_DESKTOP_PORTAL_DIRS would have been better).
• Otherwise,
  • if XDG_DATA_DIRS is set, look into them with suffix /xdg-desktop-portal/portals.
  • Othersise, look into /usr/local/share and /usr/share as per the spec, with suffix /xdg-desktop-portal/portals.
  • Finally look in ${datadir}/xdg-desktop-portal/portals just like D-Bus.

And quite honestly, with this logic, XDG_DESKTOP_PORTAL_DIR can be dropped all together, but that would be a breaking change.

WDYT?

[podiki]

I like this idea, can keep XDG_DESKTOP_PORTAL_DIR around, but using the standard XDG directories makes sense. Related on this front, see the issue I just filed #616 (doesn't handle multiple directories).