Textlayout

Something like that is needed. 

For the GtkTextLayoutBackendHandler it would be better if the backend wouldn't just be a Pango.Layout. It needs a mapping between the utf-8 byte index and char index coodinates because pango uses always byte index coodinates.
The index transformation functions are time critical therefore it shouldn't be done inside these functions. 

It's not really important to have that in v1 - but should be added in the future.