Apple: Use namespaced category name sandbox helper

Pick-to: 6.5
Fixes: QTBUG-119338
Change-Id: I8a62b3fff0c1f3de2b6ae3332f6a1ecc7b1561e5
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
(cherry picked from commit c8cd9017f6c9696d79c3e7b7f0b71748839c263f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qcore_mac.mm

@@ -505,7 +505,7 @@ bool qt_apple_isSandboxed()
 }
 
 QT_END_NAMESPACE
-@implementation NSObject (QtSandboxHelpers)
+@implementation NSObject (QtExtras)
 - (id)qt_valueForPrivateKey:(NSString *)key
 {
     if (qt_apple_isSandboxed())

src/corelib/kernel/qcore_mac_p.h

@@ -204,7 +204,7 @@ Q_CORE_EXPORT bool qt_apple_isSandboxed();
 
 #if defined(__OBJC__)
 QT_END_NAMESPACE
-@interface NSObject (QtSandboxHelpers)
+@interface NSObject (QtExtras)
 - (id)qt_valueForPrivateKey:(NSString *)key;
 @end
 QT_BEGIN_NAMESPACE