Fix compatibility with legacy runtimes.

This fixes compilation for 32-bit Mac apps, as they use the "legacy"
runtime. In our context it means:

 - Don't use hash literals.
 - Explicitly @synthesize properties.
 - No ivars in class extensions.
 - Forward declare more methods.

See this link for more details of what that entails:
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtVersionsPlatforms.html
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