fix(panel): hide empty monitor groups

[VictorBenitoGR]

Summary

Fixes a panel spacing regression introduced in version 27 where disabled resource groups still reserved horizontal space in the GNOME top bar.

When only one item is visible, for example RAM usage, the indicator could still include spacing for hidden groups like CPU, swap, disk, network, and GPU. This made the panel item much wider than its visible content.

This change keeps the existing item order behavior, but hides whole panel groups when none of their metrics are visible.

Type of change

• Bug fix
• New feature
• Refactor
• Documentation only
• Translation update

Validation

• make validate passes locally.
• I tested this change in GNOME Shell.
• Tested on GNOME Shell version(s): 46
• Tested on Ubuntu 24.04.4 LTS, Wayland.
• Verified the RAM-only indicator no longer reserves empty space after disabled groups.

Documentation and i18n

• No documentation changes needed.
• No translatable strings changed.

GNOME consistency checklist

• UI behavior remains coherent with GNOME conventions.
• Preferences follow existing GTK4/Libadwaita patterns.
• Schema/settings changes are backward compatible.

Linked issues

Fixes #129

Target branch

develop