Improve linkages between Keypads and Fading doors

[Lexicality]

@Jelman21 in #6
enable to create fading door and link multiple keypads to that fading door, without the setting the button nonsense (like the other / original keypad tools)
or, do some crazy crap to allow multiple keypads to be wired to a single door via wiremod

Thoughts:

• Add more wire inputs for fading doors
• Add a lua trigger for keypads
• Make fading doors able to hook into them?