InstanceDecorators of URichTextBlock is increased everytime add it to viewport

URichTextBlock::UpdateStyleData() does not check whether DecoratorClasses already had any specificed DecoratorClass. So every time you turn UMG on and off from the Viewport, Decorator Classes will increase indefinitely.

I propose that we add check code to a following code ( ex. URichTextBlock::GetDecoratorByClass )

void URichTextBlock::UpdateStyleData()void URichTextBlock::UpdateStyleData()
{ 
...

if (!ResolvedClass->HasAnyClassFlags(CLASS_Abstract))
{ 
   URichTextBlockDecorator* Decorator = NewObject<URichTextBlockDecorator>(this, ResolvedClass); 
   InstanceDecorators.Add(Decorator); 
}

1. Create DecoratorClasse based on URichTextBlockImageDecorator
2. Create WBP
3. add RichTextBlock to 2
4. set 1 to DecoratorClasse of 3
5. PIE
6. call Add to viewport to 2 
7. call Remove from parent to 2
8. call add to viewport to 2

result : DecoratorClasses of RichTextBlock has 2 instances.