lib/cocoa: intro the Hello World example in Cocoa

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/cocoa: intro message box example in cocoa

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/cocoa: intro an example with extern Objective-C classes

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>