a11y: Use "presentation" role instead of "hidden" state

a11y: Use "presentation" role instead of "hidden" state

The intended use case of the "presentation" role is

| […] when an element is used to change the look of the page but does not
| have all the functional, interactive, or structural relevance implied
| by the element type, or may be used to provide for an accessible
| fallback in older browsers that do not support WAI-ARIA.

One of the examples is, literally:

| An element whose content is completely presentational (like a spacer
| image, decorative graphic, or clearing element);

Which fits perfectly for the GtkTreeExpander's "indent" builtin icon
widget.