cling based C++ as scripting language / hot code reload

cling based C++ as scripting language / hot code reload
Why? Able to run C++ script in runtime or compile it for max speed ( as in example https://github.com/derofim/cling-cmake )

> **HOT code reload**
> possible approaches:
>
> store app state
> fix cling undo for files
> https://root-forum.cern.ch/t/loading-unloading-class-as-interpreted-macro-in-cling-multiple-times/32976/2
>
> execute cling code to change callbacks & variables
> nested cling::Interpreter with multiple cling::MetaProcessor
> IDK how to do it, but you can create child cling::Interpreter

No repository files or tests are named. Start by reviewing the cling-cmake example and the linked cling discussion, then trace how a cling::Interpreter, MetaProcessor, application state, callbacks, and variables could fit into ouzel. Done would require an agreed design and documented scope for runtime C++ scripting and hot code reload.

Written by the indexing model from the issue text.