Add wxOSX_10_6_AND_LATER macro and use it.

Add wxOSX_10_6_AND_LATER macro and use it.

Replace all occurrences of

	@interface wxFoo : NSFoo
	#if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_6
	    <NSProtocol>
	#endif

which we used for protocols new in 10.6 with just

	@interface wxFoo : NSFoo wxOSX_10_6_AND_LATER(<NSProtocol>)

which looks better and simpler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62755 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775