RichTextLabel's bbcode_text property is not tag-stack-aware.

[willnationsdev]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Windows 10
Godot master

Issue description:
If you manually add content to RichTextLabel's internal tag stack using append_bbcode and the push_*/pop functions, I would expect to be able to print bbcode_text and see the corresponding opening and closing tags visible in the output (at least, as much as is possible. I know you can't perfectly do that with push_meta(Variant), but for most you could). To the end user, they don't even see the internal tag stack, so when they use append_bbcode, they THINK that they are in fact editing bbcode_text...except that they aren't...<_< We should try to maintain that illusion.

Steps to reproduce:
Start a new scene with RichTextLabel as the root. Toggle on the bbcode bool. Add a script:

# RichTextLabel.gd
func _ready():
    add_text("hello")
    print(bbcode_text) # prints nothing to the screen

I would have expected to see "hello" printed as a result of this.

Also, fixing this Issue would probably also fix #12881.

[KoBeWi]

Still valid in 3.2.4 beta2