UserItem: GTK4 prep

[danirabbit]

Review with hide whitespace changes

• Inline namespace
• code style
• Use child property
• explicit margins
• Use style class for small label
• Use child property
• Way less complicated way to add on the "(Administrator)" label

[jeremypw]

@danirabbit Conflict needs fixing.

[danirabbit]

@jeremypw fixed!