WidgetResource: rewrite and cleanup

[danirabbit]

Can be rebase merged if desired. GTK4 prep, cleanups

• set margin in usage-label-container CSS. You must ninja clean to rebuild CSS
• set_main_chart () → main_chart property
• Initialize widgets in construct block
• Replace Grid with Overlay and Box where appropriate
• Code style. Explicitly set margins
• Replace ToggleButton with MenuButton

LabelVertical is only used inside WidgetResource, so absorb it there

• Clicked signal is never used
• Use a box instead of a grid
• css class is never used
• label_vertical_main_metric → main_metric_value

[stsdc]

love it