action: Add DragAction, an action implementing drag capabilities

action: Add DragAction, an action implementing drag capabilities

DragAction is an Action sub-class that provides dragging capabilities to
any actor. DragAction has:

  • drag-begin, drag-motion and drag-end signals, relaying the event
    information like coordinates, button and modifiers to user code;

  • drag-threshold property, for delaying the drag start by a given
    amount of pixels;

  • drag-handle property, to allow using other actors as the drag
    handle.

  • drag-axis property, to allow constraining the dragging to a specific
    axis.

An interactive test demonstrating the various features is also provided.