`IntoAtoms` support for `ComboBox`

`ComboBox` still uses `WidgetText` rather than `IntoAtoms`.
I would like to display little images next to the options in my dropdown list, as well as in the "selected text".

My current workaround is to use a `MenuButton` instead, which does support `IntoAtoms`.

Start by locating the ComboBox implementation and its use of WidgetText, then compare it with MenuButton's IntoAtoms support. Verify how IntoAtoms is used for dropdown options and selected text. Done means ComboBox accepts IntoAtoms and supports images in both places, with relevant tests updated or added.

Written by the indexing model from the issue text.