Fixed an issue with dropping elements outside regions

[sunpietro]

Problematic use case:

1. Drag an element,
2. Move onto a dropzone,
3. The placeholder appears,
4. Move outside the dropzone,
5. Drop element,
6. The element is dropped on a dropzone.

Expected result:

The element should not be dropped. No change should occur.

Improved:

• started listening to mouseup/touchend events when a user starts dragging an element,
• clearing dragster regions when the onBeforeDragEnd callback returns false.