dnd: Use GtkGestureDrag to initiate DnD
dnd: Use GtkGestureDrag to initiate DnD

This gesture is used by gtk_drag_source_set() to determine
whether dragging moved past the threshold. The gesture events
are handled via the usual ::event callbacks, so we don't mess
up with callers expecting that to happen in a signal handler.

If the sequence gets claimed somewhere else in the event widget
stack, the DnD gesture will be cancelled.

chergert/gtk,Adamovskiy/gtk,Adamovskiy/gtk,chergert/gtk,jessevdk/gtk,jigpu/gtk,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,alexlarsson/gtk,chergert/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,Adamovskiy/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,ahodesuka/gtk,chergert/gtk,Sidnioulz/SandboxGtk,jadahl/gtk,jessevdk/gtk,jessevdk/gtk,Lyude/gtk-,alexlarsson/gtk,ahodesuka/gtk,Adamovskiy/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,Lyude/gtk-,davidgumberg/gtk,jadahl/gtk,ahodesuka/gtk,Lyude/gtk-,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,davidgumberg/gtk,Lyude/gtk-,Lyude/gtk-,chergert/gtk,ahodesuka/gtk,Adamovskiy/gtk,grubersjoe/adwaita,davidgumberg/gtk,ahodesuka/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,ahodesuka/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,jessevdk/gtk,grubersjoe/adwaita,Lyude/gtk-,jadahl/gtk,jadahl/gtk,Adamovskiy/gtk,davidgumberg/gtk,ahodesuka/gtk,jessevdk/gtk,Adamovskiy/gtk,jessevdk/gtk,davidgumberg/gtk,grubersjoe/adwaita,Lyude/gtk-,jadahl/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,Adamovskiy/gtk,Lyude/gtk-,alexlarsson/gtk,jadahl/gtk,chergert/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,jigpu/gtk