Fix: the gtk expander widget bugfix

[ovalkonia]

Description

As it was mentioned in this issue -> #1085, if the name property was set for the expander widget, then child elements didn't show up. Essentially it was a choice between a labeled but child-free expander or labeless but with children expander.

This behavior came from the build_builtin_gtk_widget() function. To be more precise, this function adds child elements only if there aren't any children that have been already added with the widget_use_to_gtk_widget() function. The problem with that in this case is that expander's label is actually considered to be a child element itself:

So now the build_gtk_expander() function handles the child element explicitly (see diff).

Fixes #1085

Usage

N/A

Showcase

Here's the minimal configuration used below:

(defwindow example
    :monitor "0"
    :geometry (geometry
        :x "0"
        :y "0"
        :width "100%"
        :height "35px"
        :anchor "top center"
    )
    :exclusive "true"
    (expander
        :name "foo"
        :halign "start"
        (label :text "expanded child")
    )
)

The result:

Works as expected

Additional Notes

Now the expander widget is also required to have exactly one child. If no children were provided, there would be an error. I just thought it would be nice, but it can be removed without any problems if necessary.

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

[elkowar]

Thanks for your work! Merged!

[ovalkonia]

@elkowar, whoops, I think I accidentally put the changelog entry under the wrong version. Should be under unreleased, my bad. Must have been sleepy when I did this. Can you fix that, please? Another pr would probably be too much

[w-lfchen]

i've added this to #1155, that pr contains some general fixes by now anyways

[ovalkonia]

Thank you!