Revert "Removed tgui::to_string as std::to_string can now be used"

This reverts commit 947ae755e7df6cd9bd734c6203dd3ec3488d2ed1.
Revert "Replaced atoi and atof by stoi and stof"

This reverts commit bab857bd5e93a2d56bff5320c063c4720fac45b1.
Added a method to access directly the gui from a container.