This allows the familiar file manager semantics of ctrl+drag = copy,
drag = move to be used when dragging and dropping windows between
activities using the taskbar and the activity pager applet.
The code to handle a window drag from the taskbar to drop on the
Activity Pager to add the window to an Activity was broken,
because the test to see if the drop location is a running activity
still treated the itemId as an integer indexing into an array of running
activity indices, rather than an id string that could be a member of the
list of running activities.