Expanded TextBuffer interface to allow borrowed values.

[Zenithsiz]

Previously the TextBuffer trait only allowed owned values, since it relied on Clone, Default and 'static.
This allows it to use borrowed values.

A possible use case for this is creating a buffer which has dynamic max length:

struct LimitedStringBuffer<'a> {
    text: &'a mut String,
    max_len: usize,
}

This wasn't possible in the previous api, as Default and Clone cannot be implemented for this struct, and it wasn't 'static.

All new trait methods have a default impl using the existing 2 methods, so this is backwards compatible and also suffers from no performance loss on Strings due to them implementing these new methods.

[emilk]

I wanted to wait with this until #412 is in, and now it is.

This seems like a good idea, but there is a conflict with the above PR: we now make a clone of the text each frame (not great, not terrible) to easily detect and report changes for screen readers. This would have to change to instead of cloning into an S to clone into a String (e.g. call text.to_string() instead of text.clone())

[Zenithsiz]

It should also be possible to reuse the buffer by storing it somewhere, which would avoid an allocation every frame, and instead just do old_text.clone_from(text.as_ref()).

I haven't implemented that however, as I'm not sure where I could store the old buffer, nor if it would be a good idea.
Have rebased on master though.