Finalize AttributedText API

[matthew-carroll]

AttributedText has some mutable and some immutable behaviors. It's a conflicting API.

Either AttributedText should commit to be mutable, or immutable, or 2 classes should exist; one for each use-case.

[salihgueler]

Maybe related to this:

When there is an AttributedText with only bold or italic in a paragraph, a person can not make an adjustment to the text and it throws the following exception.

Exception: Another AttributedSpans can only be appended after the final marker in this AttributedSpans. Final marker: [SpanMarker] - attribution: italics, offset: 0, type: SpanMarkerType.end

STR:

• Create a sentence in a paragraph with Bold (or italic, up to you) characters.
• Save it (serialize it)
• Remove all the words/characters except the bold one
• Save it again.
• Try to add characters to the start (or end) of the bold word, it does not work.