Shell dialog styling

We should align the overall styling/layout of system dialogs in Phosh with desktop GNOME Shell (as much as possible), since those are being redesigned right now.

These are the desktop mockups: https://gitlab.gnome.org/Teams/Design/os-mockups/tree/master/system-dialogs

The overall layout is more or less that of a message dialog, with

ultra-bold title
centered title and description
no symbolic icon on the left
dark gray (not black) background

Examples:

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Admin message

Shell dialog styling