hardcode triangulate sectors
This adds "Triangulate Sectors" to the list of sector actions, directly into the builder. The UDB Script triangulateSectors already does this, but with the current state of equation slopes and UDMF vertex slopes, I think integrating this directly into the builder (preferably with a default keybind like Z! if I can figure that one out) seems like an obvious next step.
This MR does not remove the script, though, because it still functions perfectly as an example.