SaveTool: make the weapon part selector focus on the current item.

src/SaveTool/SaveTool_MassViewer_Weapons.cpp

@@ -479,6 +479,9 @@ void SaveTool::drawWeaponEditor(Weapon& weapon) {
                         if(ImGui::Selectable(mapped_part.second.data(), mapped_part.first == part.id)) {
                             part.id = mapped_part.first;
                         }
+                        if(mapped_part.first == part.id) {
+                            ImGui::SetItemDefaultFocus();
+                        }
                     }
 
                     ImGui::EndListBox();