Set configuration dynamically

[obraunsdorf]

Currently all fields in src/util/pbcConfig.h are hard-coded.
It would improve usability, if the configurable values could be set by the user through the GUI and saved across executions.

[obraunsdorf]

I thought about the following solution to this:

1. Draft a TOML[1] or JSON[2] file syntax to map to all configurable fields of pbcConfig.h
2. Add GUI options to save and load the current configuration to/from File

I experimented with implementing this in Rust but have not come far due to time limitations. Help would be very appreciated :)
I could push my current working branch for this which already implements Rust bindingst to a new PBCRustConfig.

[1] https://de.wikipedia.org/wiki/TOML
[2] https://de.wikipedia.org/wiki/JavaScript_Object_Notation