Various updates/modernizations/improvements to SimpleCocoaVTK example

- Now require newer Xcode and more modern Objective-C dialect.
- Now require building VTK with VTK_CUSTOM_LIBRARY_SUFFIX="",
  which is best practice anyway if using Xcode, so that the CMake-built
  VTK libraries don't constantly change filenames (by having the version number embedded).
- Adjusted Xcode project paths to remove version from filename.
- Added a window controller class, which is more idiomatic.
- The example now uses ARC (automatic reference counting) memory management by default.
- renamed ivars to start with underscore, per Cocoa convention
- added project.xcworkspace to .gitignore
- added usual VTK divider line comments