Multiple text properties/dynamic text input?

[s5bug]

Following up from #174, as I can't find the utflib-and-attrstr branch, what if we were able to create some sort of input where certain parts of text were editable, styled, etc and others weren't in one sort of "text buffer"? (A.k.a. how to create an emacs-style buffer system with libui)

What would might this code look like in the libui style? What would it take to implement?