Added callback to Process

Added a callback to Process, which is executed once the process is done.

It takes the block's result as a parameter, which may eventually be null if the block is not a reporter.

You can test it by adding a test menu item to *BlockMorph.prototype.userMenu*, such as:

```
menu.addItem(
	"callback test",
	function() {
		var top = this.topBlock(),
			receiver = top.receiver(),
			stage;	
		if (top !== this) {return; }
		if (receiver) {
			stage = receiver.parentThatIsA(StageMorph);
			if (stage) {
				stage.threads.stopProcess(top);
				stage.threads.startProcess(top, false, false, function(result) { alert(result) });
			}
		}
	}
);
```