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/ApplicationInfoDropTarget.java

@@ -110,6 +110,10 @@ public class ApplicationInfoDropTarget extends ImageView implements DropTarget,
         }
     }
 
+    public boolean isDropEnabled() {
+        return true;
+    }
+
     public void onDragEnd() {
         if (mActive) {
             mActive = false;