Clone stamp improvements

1) When using a different layer as the source, make the tool not to reset the source to the paint layer every time you alt-click for a new source location.

2) If you have done some cloning, done some other stuff, and then go back to cloning, allow new strokes to be added to the previous application of the paint effect, rather than creating paint 2, paint 3, &c.

As an aside--make the forum searchable to isolate program suggestions, so I don't have to go through 266 posts asking why the clone brush doesn't work in order to see if someone has previously suggested these.