Add xdg-toplevel-drag protocol

This protocol allows applications to request that a window is moved at the same time as a drag operation - effectively dragging windows. With this features such as detaching a tab from a window and reattaching it, dragging tabs between windows or (un)dockable tool windows can be implemented. Based on the previously proposed extended drag protocol but trimmed down.

Signed-off-by: David Redondo kde@david-redondo.de

The previous proposal was !46 (closed),

Requirements for merging

• Review: @zzag
• Implementations
  • Qt
  • KWin
  • Chrome & Exo
• ACKs from members
  • Mir: @RAOF
  • KDE: @davidedmundson
  • Qt: @eskilblomfeldt
  • GNOME: @carlosg

Videos showing the Qt and KWin implementation in action:

topleveldrag qtworking

Chrome in KWin:

chromiumworking