Project Config reform

[ussserrr]

• [ ] Consider PlatformIO-like schema: only explicitly stated options should be stored in the actual config file
• [ ] Use getters like getboolean/getint (no more yes_options/no_options/..., but need to check for backwards compatibility with older configs)
• [ ] Add prepending \n for pretty displaying
• [ ] Implement optional global config (like ~/.stm32pio.ini, or ~/.platformio/stm32pio.ini, or via Qt QSettings if GUI version has been used...)