fix(panel): avoid iconless layout spacing

[VictorBenitoGR]

Summary

Fixes iconless panel alignment so single-item displays stay visually centered.

When monitor icons were disabled, a simple display such as RAM percentage only could appear shifted to the left inside the GNOME top bar pill, leaving a small but noticeable extra space on the right.

This change keeps icons working normally when enabled, but removes icon actors from the panel layout when icons are disabled so they no longer affect alignment. It also keeps the spacing between multiple iconless monitor sections consistent with the previous visual spacing.

Note: this PR is stacked on top of #130 because it adjusts the panel group layout introduced there. It should be reviewed after #130.

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.
• Tested icons disabled with RAM-only panel display.
• Tested toggling icons on and off keeps icons working when enabled.

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

Related to #129

Stacked on #130

Target Branch

develop