On X11 we need to keep the QDrag around a little longer.
Right now, drag is scoped local to launchDrag() and will be destroyed
immediately when launchDrag() finishes. However for X11 we must keep the
drag object alive a little longer. It may only be destroyed once the
drag operation has been accepted by another window, otherwise drag and
drop is broken on Linux and no action happens when something is dragged
from Telegram into another window.
This is because there is still communication happening in the XDND
protocol once QDrag::exec() finishes. See the documentation for
reference: https://freedesktop.org/wiki/Specifications/XDND/
Fixes #17291
