Command to snap spiders on to the wire

[RazinShaikh]

Suppose we have a lot of phase gadgets and we want to connect the top row of spiders together through a wire. We then need to connect each spider to the next threading our way from left to right. This is very tedious. It would be useful to have a shortcut which can connect the selected spiders to the wire below it.

[jvdwetering]

As a suggestion for how this could work: when we have the "merge nodes" command from Tikzit implemented #253, then holding the m key while dragging an edge should connect the edge through all the spiders you pass in the order that you pass them.
Although it might be a bit difficult to drag a straight line for this long, so perhaps the original suggestion for just merging everything the line passes through is better.

[jvdwetering]

Implemented by #337