Rewrite of `Key` structure

[gedankenexperimenter]

Rewrite of the Key structure as a class instead of a union. This introduces a new set of classes, one for each of the core Key types:

• Key::Keyboard
• Key::Consumer
• Key::System
• Key::Mouse
• Key::Layer
• Key::Plugin

I'm ambivalent about the names, but I still feel it's the least bad option.

[gedankenexperimenter]

After some optimizing, this branch is now smaller than the version with the Key union by 138 bytes.