Accommodate macOS 11.0 Big Sur in gdk_quartz_osx_version.

This may need to be changed next year, it depends on what Apple
decides to do with version numbers going forward.

Also updated GdkOSXVersion to include Catalina and Big Sur.

gdk/quartz/gdkglobals-quartz.c

@@ -42,6 +42,8 @@ gdk_quartz_osx_version (void)
 
       version = [[NSProcessInfo processInfo] operatingSystemVersion];
       minor = version.minorVersion;
+      if (version.majorVersion == 11)
+        minor += 16;
 #endif
     }
 

gdk/quartz/gdkquartz.h

@@ -40,7 +40,9 @@ typedef enum
   GDK_OSX_SIERRA = 12,
   GDK_OSX_HIGH_SIERRA = 13,
   GDK_OSX_MOJAVE = 14,
-  GDK_OSX_CURRENT = 14,
+  GDK_OSX_CATALINA = 15,
+  GDK_OSX_BIGSUR = 16,
+  GDK_OSX_CURRENT = 15,
   GDK_OSX_NEW = 99
 } GdkOSXVersion;