Implement a way to map entities to their names, which comes up often enough.

src/guecs/ui.cpp

@@ -38,11 +38,19 @@ namespace guecs {
     auto ent = entity();
     // this lets you look up an entity by name
     $name_ents.insert_or_assign(name, ent);
+    // this lets you get a name by entity
+    $ents_name.insert_or_assign(ent, name);
     // this makes it easier to get the name during querying
     set<CellName>(ent, {name});
     return ent;
   }
 
+  const std::string& UI::name_for(Entity gui_id) {
+    assert($ents_name.contains(gui_id) &&
+        "Attempt to get name_for an Entity but that's not a valid ID.");
+    return $ents_name.at(gui_id);
+  }
+
   Entity UI::entity(const string& name) {
     assert($name_ents.contains(name) &&
         "GUECS entity does not exist. Mispelled cell name?");