Draft: Joint auto selection when using "Stroke Selection" option.

This MR contains the code for joint auto selection when using "Stroke Selection" option.

Since the selection is made of array of GimpBoundSeg this will check if angle between any two of the adjecent segments is less then 120 degree (selected arbitrarily). As when selection is created, all the segments are either vertical or horizontal, so angle between two adjecent ones is always 90 degrees. So, instead we consider two adjecent segments as one while checking.

Please check the code for both coding guidelines and memory leaks.