TextEdit Atom prefix/suffix (#7587)

* part of https://github.com/emilk/egui/issues/7264
* part of https://github.com/emilk/egui/issues/7445

This PR changes the layout within the TextEdit to be done with
AtomLayout. It also adds a Prefix and Postfix that allows adding
permanent icons / icon buttons within the textedit.

Breaking changes:
- Removed `TextEdit::hint_text_font`. Hint text is an atom now, so the
font can be set that way

<img width="264" height="130" alt="Screenshot 2025-10-06 at 12 25 21"
src="https://github.com/user-attachments/assets/03b6b56b-ca82-4ac3-b5c0-585cca336834"
/>

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>
Co-authored-by: lucasmerlin <8009393+lucasmerlin@users.noreply.github.com>

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