xdg drag and drop portal support

- adds a new function exportUrlsToPortal that gets called by the sender
to extend qmimedata with transfer context (and send the relevant files
off to the portal)

- extends urlsFromMimeData to unpack the transfer context and redirect
the urls to the ones received from the portal

main disadvantage: this would bring in qdbus as optional dependency of
kcoreaddons along with runtime depends on portals and kio-fuse (albeit
not required)

main advantage: receivers would hopefully already call urlsFromMimeData
saving us from having to change every single drag receiving application
(we still have to change every drag sending application though :(()