assistant: use symbolic icon instead of label for back buttons

Use a symbolic icon instead of a label for back button in the Download OS dialog. This change improves compliance with the GNOME Human Interface Guidelines ("The most common usage for UI icons is to identify buttons. Here, it is often preferable to use an icon over a label [...] such as search, menu, forward, back and share") and solves issue #893 (closed).

Also, move the call to populate_recommended_list () after the button appearance is updated (from "Cancel" label to "go-previous-symbolic" icon), so the button changes as soon as the Download OS dialog appears instead of waiting until the entire list loads.

Here are some screenshots of how it looks.

State	Screenshot
Before (label)
Now (icon)

Thanks for developing Boxes! 🚀