removing an element by dragging does not trigger the 'removed' event

Hi,

Not sure if this is a bug or intended behaviour, but dragging an element outside the grid to delete it using the 'removable: true' option, does not trigger 'removed' event. 
Relevant code seems to be [here](https://github.com/troolee/gridstack.js/blob/master/src/gridstack.js#L1156). 

If this is intended behaviour my two cents are that it should trigger an event :) 