Removed the automatic "link swapping" feature from the node link operator: When a link was being dragged to an already connected input, the existing links were shifted to the next free socket. This was
originally intended as a way to speed up workflow for math and mix nodes, but more often than not it just gets in the way. Most binary (or n-ary) functions are not even commutative, i.e. changing the
order of sockets does not usually produce the correct result. Also this includes the more common case where one actually wants to replace a socket, which then requires a second click to remove the
shifted connection. All in all this is not a helpful feature.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61178 954f8c5b-7b00-dc11-b283-0030488c597c