Added gtk stack widget

[vladaviedov]

Description

This PR adds the stack widget from GTK.
This widget shows one of its children at a time.

Usage

(stack
    :selected int_var

    (child 0)
    (child 1)
    (...)
)

Setting int_var will show child with respective index.

Showcase

Additional Notes

I made this as a workaround to the problem I was having in #835, however I don't think it resolves that issue.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing