Summary:
The main difference compared to version 2 is additional drag and drop
actions. The source and destination can negotiate whether the data
should be copied or moved or the user should be asked for either or.
This seems to be important for GTK, but is not yet implemented in Qt.

The main motivation for adding support is that it is required by SDL to
launch on Wayland.

BUG: 386993

Test Plan: Extended test case, sdl apps now start

Reviewers: #frameworks, #plasma, #kwin

Subscribers: plasma-devel

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D9136