VLCDefaults

This MR is a proposal to remove direct use of UserDefaults, and instead access values through a wrapper VLCDefaults.

Pros:

• Allows simple dot notation to get and set values.
• Defaults which are accessed with composite keys use reliable functions instead of string building in various places in the codebase
• AppDelegate no longer needs the big dictionary in it
• Ownership is in Swift; Objective-C is a client instead of the other way around
• not included in this MR: objects can be serialized and deserialized reliably using swift Codable.
• not included in this MR: can be made observable, possibly even on a per-property basis.

Cons:

• Causes churn.
• No user facing benefit.
• Potential for mistakes during refactor

If this is wanted, I will continue.