Adding support for drag and drop of folders and shortcuts.

also:
- Long press on empty space on workspace now brings up customization tray
- Fixed: while dragging, items appeared to be dropping on folders two cells to the right
- Fixed: Disabling drops on folders when the workspace is shrunken
- Fixed: account for scaling of dragged items when checking if they overlap
          with shrunken workspace screens
- Making folder icons dimmable to match shortcuts and widgets
- When deciding with shrunken workspace screen we're dragging to, we now use the closest screen rather than the one that has been overlapped the most
- Refactored drag/add mechanism, removing array of occupied cells from CellInfo
- Removed dead code/variables

src/com/android/launcher2/DropTarget.java

@@ -23,6 +23,12 @@ import android.graphics.Rect;
  *
  */
 public interface DropTarget {
+    /**
+     * Used to temporarily disable certain drop targets
+     *
+     * @return boolean specifying whether this drop target is currently enabled
+     */
+    boolean isDropEnabled();
 
     /**
      * Handle an object being dropped on the DropTarget