Implement serialization of keyboard shortcuts

[AaronVanGeffen]

This crude (de)serialization of keyboard shortcuts to the OpenLoco config files, aiming to fix #1135 and allowing for easy extension in the future.

To do:

• Config saving and loading, using SDL serializer for names
• Declare defaults for all existing shortcuts and use when missing
• Check against new shortcut definitions in Keyboard.cpp
• Update keyboard shortcut window to reflect the new list, while still allowing localisation
• Shortcuts should be bound to the new declarations, not the old ones
• Allow rebinding the NumPad Enter key separately (following up on Make keypad enter work the same as normal enter #380)