Githawk: preferredContentSizeCategory called on background thread

[BrianLitwin]

Repeat of #1225.. as stated in original thread, not an urgent issue, just throwing a "Main Thread Checker: UI API called on a background thread" warning in XCode.

The preferredFont property that's causing the warning is accessed here and then here in NotificationCell, from this function.

[rnystrom]

I guess those are being evaluated lazily? Is that how static works in Swift?

_{Sent with GitHawk}

[BrianLitwin]

yeah that must be it, calling

let _ = NotificationCell.inset 

anywhere on the main thread silences the warning

[ismetanin]

@BrianLitwin @rnystrom
Fixed #2037