Drag'n'Drop support in the mouse engine code

Hotkey icons can now be dragged along the bar for easy reassignment
Talents can be dragged from the use talents window into the hotkeys
Items can be dragged from the inventory window into the hotkeys


git-svn-id: http://svn.net-core.org/repos/t-engine4@3998 51575b47-30f0-44d4-a5cc-537603b46e54