Modify textarea's text
Hello, I've been having fun with this library. I'm currently trying to modify specific lines in textarea.text. Basically what I'm doing is prepending some lines wtih tabs to simulate that persistent tab effect on Enter. So far so good, I can get the line by counting characters against cursor position, but when I try to `textarea.text = "something"` I get the following error:

```
(dirfun:12443): Gtk-WARNING **: 19:03:49.532: Invalid text buffer iterator: either the iterator is uninitialized, or the characters/pixbufs/widgets in the buffer have been modified since the iterator was created.
You must use marks, character numbers, or line numbers to preserve a position across buffer modifications.
You can apply tags and insert marks without invalidating your iterators,
but any mutation that affects 'indexable' buffer contents (contents that can be referred to by character offset)
will invalidate all outstanding iterators
```
So I'm wondering what would be the correct way to do this. I can't find any method in the NiGui source that might handle this, so not sure if it's already implemented. I tried running the text assignment on a thread, but it complained about GC limiations.