This adds `WidgetType::Image` and correctly sets it in the Image widget.
This allows us to query for images in kittest tests and tells accesskit
that a node is an image.
It also adds `Image::alt_text` to set a text that will be shown if the
image fails to load and will be read via screen readers. This also
allows us to query images by label in kittest.
* [x] I have followed the instructions in the PR template
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Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>
