extern_methods! shorthand for property access

[madsmtm]

Some classes have a lot of simple properties; it may be interesting to have an easier way of accessing these?

Examples:

• UIColor has a lot of class properties that return a specific UIColor instance
• UIView has a lot of getter/setter properties
  • Difficult because some cases might use &mut self for mutation, while others might use &self
• The x -> isX translation for BOOL properties can be kinda confusing, and would be nice to handle automatically

See also #161 (comment).

[madsmtm]

Since we're autogenerating most extern_methods! declarations via. header-translator these days, I don't think it makes much sense to work towards a feature like this anymore - users can always define their own helper macro if they feel the need.

Also, extern_methods! still looks like fairly straightforward Rust syntax, which I doubt can be said of the syntax properties would have.