Drag-and-drop support

[Syncaidius]

Implement Drag-n-drop support to the UI system:

• OnDrag() method - Triggered when dragged. Can return false to reject dragging
• OnDropped(UIElement target) method - Triggered when the UIElement is dropped on another UIElement
• OnDragOver(UIElement over) - Triggered when another UIElement object is over the current one. Can return false to disallow

Dropping:

• OnDropReceived(UIElement dropped) - Triggered when an UIElement receives a drop action from another UIElement. Can return false to reject the dropped object