Enhance iOS export

- The export process now builds complete .ipa on macOS, instead of just
   creating XCode project.

 - The project includes Capabilities games usually require: Game Center,
   Push Notifications, In-App Purchase.

 - Icons and launch screens can be specified in export preset.

modules/gdnative/include/gdnative/gdnative.h

@@ -49,8 +49,8 @@ extern "C" {
 #elif defined(__APPLE__)
 #include "TargetConditionals.h"
 #if TARGET_OS_IPHONE
-#define GDCALLINGCONV
-#define GDAPI
+#define GDCALLINGCONV __attribute__((visibility("default")))
+#define GDAPI GDCALLINGCONV
 #elif TARGET_OS_MAC
 #define GDCALLINGCONV __attribute__((sysv_abi))
 #define GDAPI GDCALLINGCONV

modules/gdnative/include/nativescript/godot_nativescript.h

@@ -51,8 +51,8 @@ extern "C" {
 #elif defined(__APPLE__)
 #include "TargetConditionals.h"
 #if TARGET_OS_IPHONE
-#define GDCALLINGCONV
-#define GDAPI
+#define GDCALLINGCONV __attribute__((visibility("default")))
+#define GDAPI GDCALLINGCONV
 #elif TARGET_OS_MAC
 #define GDCALLINGCONV __attribute__((sysv_abi))
 #define GDAPI GDCALLINGCONV