[RTL] Keep tag stack between append_text calls.

[bruvzg]

Fixes #97965
Fixes #37774

Allow subsequent append_text calls to close tags opened by previous calls.

Note: push_* / pop can be used in between append_text calls, but not mixed:

Examples:

	# This will work.
	rtl.append_text("norm [b] bl");
	rtl.push_italics()
	rtl.add_text(" bi ")
	rtl.pop()
	rtl.append_text("bl [/b] norm")

	# This won't.
	rtl.push_italics()
	rtl.add_text("ital")
	rtl.append_text("[/i]norm")

[dustdfg]

I am not sure if there were some changes in richlabel since 4.2 and 4.3 but I think this pr also should be cherripicked if merged

[AThousandShips]

I'd say this is an enhancement and a disruptive change so it isn't appropriate for cherry picking, it isn't a pure bug fix

[KoBeWi]

Works correctly.

[akien-mga]

Needs rebase.

[Repiteo]

Thanks!