Should be able to drag Dock to tear out of window

[campagnola]

Currently, docks must be double-clicked to tear out to their own window, then dragged to rejoin with the original window. This is not intuitive; we should be able to drag in both directions.

[2xB]

There is a "simple fix", with the consequence that the drag event can no longer be cancelled. It works like this:
DockDrop.dropEvent has to accept the event by ev.accept(). This allows the following modification of Dock.startDrag:

        action = self.drag.exec_()
        # Check if dropped outside or cancelled
        if action == 0:
            self.float()

(after ev.accept(), action is 2)

This solution can be easily put in a PR, if it is okay to loose the ability to cancel the drag event in favour of the possibility to drag a dock outside. So - is it?