Letterboxing preferences UI

As discussed here and here, some user might prefer to have the letterboxing content vertically centered.

It has been suggested that this should be an user-accessible option, under a "Letterboxing" preference section including also an "Always remember last window" toggle.

Backend work:

• Letterboxing centering #41917 (closed)
• Remember last window option #41918 (closed)

Figma Design

Initial comments:

@ruihildt

don't think the letterboxing setting belongs to the "Privacy and security" section: I suggest we move it to "Website and appearance".

@ma1

1. would prefer to name the thing to be centered "web content" or "web page" instead of "website"
2. What about having a "What's this" tooltip and/or a question mark cursor when hovering the letterbox area, which would open the preference panel if clicked?

Intentionally omitting the release target (the parent issue has 13.5), leaving it to @duncan & @richard .

/cc @thorin