Make git clone cancellable

#1874 #1562 

1) Add cancel option to the clone dialog.
2) Once a user hits clone, both dialog close and clone buttons are disabled
3) The text on clone button changes to "cloning...."
4) On hitting the stop button, text changes to "Cancelling..."
5) The cloneJob itself listens for cancel event and throws an appropriate exception, if the job was cancelled and does proper cleanup.
6) The success / error is reported as the status.