Refactor Gate tool to use more WireToolObj stuff

The gate tool seems to have gone mostly neglected. WireToolObj has some great stuff that simplifies implementation of these things.

The Gate tool now uses WireToolObj's default left-click and ghost functions, which means that `sv_wirestools.lua` can now be chucked.

I'm fairly confident that this works, but because it's such a central part of Wiremod (or at least it was before E2 happened) breaking it would be very bad. Can somebody else (@Nebual?) at least read through this briefly and pull if it's okay?