`ControlTextbox` multiline overhaul, use in script editor

[Vankata453]

This PR overhauls ControlTextbox so it supports multiple lines.

A new derived from ControlTextbox class has been added - ControlScriptbox. It features suggestions when typing (just like the console does when "Tab" is pressed). Additional information and parameters for the suggested functions are also displayed. That information is fetched from the "data/scripts/reference.stsr" file, which is of type "supertux-scripting-reference". Other such files can also be imported using the register_scripting_reference() and unregister_scripting_reference() scripting functions.

Short video showcase from 2023 development summary video

NOTE: This is a WIP, since it needs to be suited to all additions and changes since the implementation of the simplesquirrel library. It also needs to further be tested for bugs.