Selective GNOME 42 conversion with automatic redundancy cleanup

[raul-lezameta]

Overview

This PR implements a paired conversion and cleanup strategy for GNOME 42 wallpaper support: wallpapers with exactly 2 images are converted to GNOME 42 syntax AND their old XML files are deleted to eliminate redundancy.

Problem Solved

Previously, updating wallpapers to GNOME 42 syntax created duplicates - the old XML file remained alongside the new one. This PR eliminates that by deleting the old files for converted wallpapers while preserving multi-image wallpapers in their original format.

What This Feature Does

• Convert: Only wallpapers with exactly 2 images are converted to GNOME 42 light/dark format
• Delete: Old XML files for those converted wallpapers are automatically removed
• Preserve: Wallpapers with 3+ images keep their original dynamic format unchanged
• Clarify: System now clearly shows which are simple light/dark variants vs truly dynamic wallpapers

Benefits

✅ Eliminates Redundancy: Old XML files deleted when converted to new syntax
✅ No Duplicates: Each wallpaper exists in only one format in the system
✅ Clarity: Simple light/dark variants distinct from truly dynamic multi-step wallpapers
✅ Preservation: Complex multi-image wallpapers retain full time-step functionality
✅ Cleaner System: Removes obsolete files, reducing clutter and confusion
✅ User Experience: Only the preferred GNOME 42 format available for 2-image wallpapers

Technical Details

• filter_wallpapers(): Selects only wallpapers with exactly 2 images
• delete_old_xml_files(): Removes old XML entries for converted wallpapers
• Early exit when no 2-image wallpapers are found
• Improved console messaging showing deletion of old files

Testing Performed

✅ Confirmed conversion of 2-image wallpapers to GNOME 42 syntax
✅ Verified old XML files are deleted for converted wallpapers
✅ Confirmed multi-image wallpapers remain untouched
✅ Tested no duplicates remain in system
✅ Validated proper file cleanup and naming conventions