The gray frame around the Greeter options needs inner margin

In feature/6882-scrollable-options, the place where the gray frame around the options is shown feels a bit ugly, and I suspect the GNOME HIG specify a greater amount of whitespace should be left between it and the widget it contains.

That’s a regression compared to 0.23~rc1.

Feature Branch: feature/6882-scrollable-options

Parent Task: #5920 (closed)

Original created by @intrigeri on 6935 (Redmine)