Improved input handling

[aleclarson]

Add the RCTWindow class that improves upon the current input handling logic.

Users can opt-in by using RCTWindow in place of NSWindow. If they don't, they'll continue to use the (flawed) RCTTouchHandler logic.

Event handling in RCTWindow is different in the following ways:

• Skip touchStart events that target a focused NSTextView

• Emit mouseOut event when the mouse leaves the RCTWindow

• Emit touchCancel event when a "touch" leaves the RCTWindow

• Support mouseMove events (which can be coalesced)

• Emit mouseMove event right after mouseUp events

• Blur the focused NSTextView when clicking outside it

• Support rightClick events

• Add altKey, ctrlKey, metaKey, and shiftKey properties to JS mouse events

• Use convertPoint:toView: to compute the relative mouse location

In the future, I might try updating RCTTouchHandler with the same logic that RCTWindow has. That's what I tried originally, but I ran into issues between NSGestureRecognizer and NSTextView.

Other changes

• rename mouseEnter to mouseOver (to align with browsers)
• rename mouseLeave to mouseOut (to align with browsers)

In the future, I might implement the mouseEnter and mouseLeave events properly (using the JS event system).