Add saving, loading, importing & exporting settings to 3D Graphics Settings Demo

[echoless3484]

Which demo project is affected:
https://godotengine.org/asset-library/asset/2738

Goal:
Adding saving, loading, importing & exporting settings with ConfigFile to 3D Graphics Settings Demo

Description:
Hey there, I am quite a beginner and i've been trying to figure out saving and loading settings (mainly loading) to my game, having a project that has this feature built in as a demo project would be very useful for those who are struggling in a similar way.

Help me:
If you know how to load settings through ConfigFile and set an existing dropdown to them, please let me know, i'd love the help! Message me on Discord if possible: "echoless" (#3484).

[Calinou]

I started to do this in https://github.com/Calinou/godot-demo-projects/tree/improve-graphics-settings-demo a while ago, but didn't have time to complete it yet.

If you know how to load settings through ConfigFile and set an existing dropdown to them, please let me know, i'd love the help!

The procedure should be the same as in the Saving games documentation. You can also check the demo project linked within that page for an example of ConfigFile usage.