Cancel autopkg run

This adds a "Cancel" button to the progress window that when pressed will cancel the current autopkg run task.
It's hidden for all other processes.

@homebysix when you get a chance would you mind taking a look at at the constraints and placement of the cancel button to make sure everything is correct.

@futureimperfect, the LGAutoPkg.h Doc Strings have also been completed, and could definitely use a proof-reader.  This may also prove an interesting test case for you to see if the autopkg's python process is terminated for you using the button (or if it has become completely detached from the NSTask).