Implement KeyCode properly

[Leo40Git]

• Add providers for KeyCode and ModifierKeyCode in DefaultGuiProviders
• Add custom (de)serializers for KeyCode and ModifierKeyCode in Gson and Jankson

Notes:

• TOML doesn't get (de)serializers, I guess TOML4J doesn't support them?

[Leo40Git]

...hm. Actually, should I have Modifier serialized by its value (single short) or by its booleans? The latter would make it more obvious what it means.

[halotroop2288]

will merge tomorrow

Or will you? 🤔