added new remove mode (like in android notification area)

Hi,

I added new way to remove items - it mimics the way in which notifications are removed from the notification area.
The item can be flinged or moved aside to be removed, and it can be done on the whole item, not just a drag handle.
 
While removing the item you can't move it up or down, and vice versa, when you're moving the item vertically, you cannot remove it. Because of that, this new mode works only with mDragInitMode set ON_DRAG (actually if you set this remove mode, the mDragInitMode is ignored and behaves as if it was set to ON_DRAG).

Additionaly the TestBedDSLV activity in demo app is modified, so it starts with this new remove mode turned on.