Add a reference document of all examples with screenshots

[Narann]

I think examples should provide screenshots to help newcomers to find in which example he should invest time. I understand some examples doesn't really need a screenshot, but I find them more engaging and they would have help me to find what I was trying to do (widget layout).

So here is this PR, it add screenshots with a reference.md document containing all the example with screenshot and tiny description. And a link to this document from the main README.md.

Screenshots generated with Xfce 4.12 "Default" style theme and apparence style "Xfce-4.6". PNG optimized with with optipng for a total of 323 KB.

Hope you will merge it and help newcomers.

[Narann]

The reference document result is visible on my fork: https://github.com/Narann/examples/blob/master/reference.md

Hope you like. :)

[ZanderBrown]

Any such screenshots should really present Gtk in as "stock" a way as possible, custom (xfce?) decorations without shadows don't really do that

Fedora or GNOME OS might be better for taking such screenshots

(side note: looks like some of the examples could be improved a tad)

[Narann]

I agree, but I had no stock Gtk available.

I still think it's better than nothing. At least, you see what the examples are supposed to show. Better screenshots could be be made later.

[Narann]

I don't know if this is related: https://nivethan.dev/devlog/rust-gtk-examples-images.html

[sdroege]

That would be great to integrate, yes. The new location of this repository is at https://github.com/gtk-rs/gtk-rs/ though.