RichTextLabel tag stack overwritten by bbcode_text manipulation.

[willnationsdev]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Windows 10
Godot 3.0 master

Issue description:
If you use append_bbcode(...), any push_* function, or the pop function to manipulate the internal tag stack of RichTextLabel, all of those changes are currently being cleared whenever someone directly edits the bbcode_text property

Steps to reproduce:
Create project with RichTextLabel root node. Add a script to it:

# RichTextLabel.gd
func _ready():
    push_image(load("res://icon.png")
    # bbcode_text += "hi"

Run the scene with and without the second line commented and you'll find that the image is missing (removed) when you manipulate bbcode_text.

[theoway]

From the documentation:
It is unadvised to use += operator with bbcode_text (e.g. bbcode_text += "some string") as it replaces the whole text and can cause slowdowns. Use append_bbcode for adding text instead.

[willnationsdev]

@theoway Is that an intended design choice though? Or would it make sense for the += operator to execute the same logic as append_bbcode?

[theoway]

@theoway Is that an intended design choice though? Or would it make sense for the += operator to execute the same logic as append_bbcode?

It does make sense for the += operator to work as append_bbcode() but the current design doesn't allow that. I don't think there's a use case for that as there's already append_bbcode().