Add UIResponder methods to ASDisplayNode

[jflinter]

This adds several UIResponder methods to ASDisplayNode, further increasing its API-compatibility with UIView. These methods include:

• canBecomeFirstResponder
• becomeFirstResponder
• canResignFirstResponder
• resignFirstResponder
• isFirstResponder
• canPerformAction:withSender

These by default just forward along to their underlying _ASDisplayView, which should already implement them correctly. The exception to this is canBecomeFirstResponder and canResignFirstResponder, which by default return NO and YES, respectively (this is so the _ASDisplayView can in turn ask its ASDisplayNode if it can become the first responder without having to expose _ASDisplayView directly).

I put this together to make it easy to close #11. However, the work's not fully done: it's still up to implementors to actually create a copyable ASTextNode subclass. This is intentional - that's pretty much the state of things with UILabel as well (see nshipster.com/uimenucontroller/ for an example of how to properly subclass and implement this). If you'd like me to contribute an ASCopyableTextNode class, I'd be happy to, although it feels a bit out-of-scope for this library.

[appleguy]

Ah, this is very interesting — I appreciate the investigation @jflinter . I'd like to merge this and very much support the general direction, but have a couple concerns:

• Have you carefully considered what will happen for layer-backed or subtree-precomposited (rasterized) nodes?
• Can you think of a way we could evaluate the performance overhead of returning non-default values for the first responder properties? It is a vague concern right now, but I worry that this could somehow increase the substantial UIKit overhead that is seen with -addSubview: and -removeFromSuperview as those operations scan the hierarchy to update the first responder.

Could you share one or two use cases that you've run into for these features? I think we should also include at least a couple unit tests for this kind of functionality, to make sure it behaves in a consistent way with layer-backing, pre compositing, and the desired behavior itself.