portal drag and drop helpers and where to put them

[Harald Sitter]

https://invent.kde.org/frameworks/kcoreaddons/-/merge_requests/212

To get sandboxed apps to work with drag and drop we need to have drags
take a roundtrip through xdg-desktop-portal and unfortunately enough
that needs to happen by passing an open fd into the relevant API,
presumably to demonstrate that the sender actually can open the file
they want to send. To that end the sender  would call a helper
function to extend qmimedata with the relevant transfer context and
the receiver needs to reverse that to get to (potentially in-sandbox)
paths again.

On top of that there's also the fact that we can drag and drop remote
sources so we kind of also need to first mount remote urls into the
file system using kio-fuse :(

All in all it's very broad in scope and requires a dependency on
qtdbus so I'm not sure kcoreaddons is necessarily the place for this
to live, at the same time we have related API here already so in the
interest of keeping related things together kcoreaddons kind of makes
the most sense. In particular since the existing helper function for
the receiver already existed meaning we get a lot of applciations to
support this without further changes.

Thoughts would be much appreciated

HS