Ui drag update

Changes to UI Drag behavior to allow making UI elements with more interesting drag behaviors (eg, multi-touch support, different behaviors for different button combinations, consistent drag cancel behaviors). A sample is also provided to demonstrate multi-touch support.

    - Multi-touch drag support
    - Combo touch support -- you get id mask of pressed buttons + number of buttons + the average of their positions (also, delta position in OnDragMove/E_DRAGMOVE).
    - OnDrag* signature updates, with more information available and improved OnDragCancel / E_DRAGCANCEL support.