Improve behaviour of "attachment mode" ring in splitter control

[RobertoBozzolo]

While defining attachment mode for controls attached to a splitter we face a very strange situation. In this image you can see on the right the aspect of the window with one control selected in "Controls on the splitter" window in Attach/Detach Controls dialog. On the right the same dialog when you click on Attachment mode control:

Now, there are two strange effects:

1. Attachment mode control is being operated while dimmed 
2. If you happen to miss Attachment mode ring, you need to select again the original control in order to have the ring enabled, which is boring and time-wasting

I suggest that at least that ring is not disabled when clicking on it or clicking away from "Controls on the splitter" box.

(this suggestion refers to CVI up to 2012SP1, I cannot check it for more recent releases)