* Use auto_connection instead of having to manually call .disconnect()
* Don't emit signals and THEN check they're empty: add if/else branch.
* Move, not copy, slots into our lists.
* Leverage emplace*() returning reference to slim down connect*First()
* Use an unordered_map of SPObject* since generally they perform best.
* Exit early instead of indenting. Use auto to declare iterators.
* Replace use of stringstream with ustring::compose(). Move away strings
* Call emptyBack() instead of duplicating its body.
* Be more const, but avoid unnecessary const_cast & copy of our items().
* Replace overly complex iterator-based loop with a range-based `for`.
* Don't copy tool->_selected_nodes->_points_list, since we only read it.
* Where we 'did' need to copy it, we can just copy the first element, as
  that's all we use, & in doing that we should be safe if list is empty.
* Clear one local vector earlier than return, to save a tiny bit of RAM!
* Add const-correct getters to node.h etc, to support const in selection
* Use constexpr in place of consts or, worse, #defines, in a few places.
* Replace another iterator-based loop with range-for to shush clangtidy.
* Only call document->getDefs() once, not per iteration of a `for` loop.