MacGui: "Stop" command modifications

MacGui: "Stop" command modifications
- Clicking on the "stop" icon in the toolbar now produces a different alert window with three choices
- " Continue Encoding " default, just dismisses the window and continues on its merry way.
- " Cancel Current and Stop " Cancels the current encode and stops the queue
- " Cancel Current and Continue " Cancels the current encode and moves to the next pending encode (if there is one).

- Also fixed an issue with deleting a canceled encode at the top of the queue list.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1718 b64f7644-9d1e-0410-96f1-a4d463321fa5