LabelRoundy: rewrite

[danirabbit]

• GObject-style construction
• Use properties instead of set functions
• Don't expose internal widgets publicly
• Use a vertical box instead of Gtk.Fixed
• Use Granite style constants
• Set mnemonic widget for screen reader

Styles:

• Rewrite styles so they're all grouped under the toplevel node
• Fix text color in dark mode
• Use tnum feature so labels don't move around as their values change
• remove redudant username-current style. This is the same as the default style

[stsdc]

Sorry for merging 'main' here. It is better if you resolve the conflicts.

[stsdc]

Not sure where, but we lost halign property. The default value should be Gtk.Align.START. Otherwise it fills the whole chart.

[stsdc]

Fixed in d058eab

The fix for halign has been commited.