Improve UX, add popover tour, metadata, and hicolor icons

- Redesign tutorial tour from modal dialogs to popovers pointing at actual UI elements
- Add beginner-friendly improvements: help buttons, tooltips, welcome wizard enhancements
- Add AppStream metainfo with screenshots, branding, categories, keywords, provides
- Update desktop file with GTK category and SingleMainWindow
- Add hicolor icon theme with all sizes (16-512px)
- Fix debounce SourceId panic in rename step
- Various step UI improvements and bug fixes

data/live.lashman.Pixstrip.desktop

@@ -5,7 +5,8 @@ Exec=pixstrip-gtk %F
 Icon=live.lashman.Pixstrip
 Terminal=false
 Type=Application
-Categories=Graphics;ImageProcessing;
+Categories=Graphics;ImageProcessing;GTK;
 MimeType=image/jpeg;image/png;image/webp;image/avif;image/gif;image/tiff;image/bmp;
 Keywords=image;photo;resize;convert;compress;batch;metadata;strip;watermark;rename;
 StartupNotify=true
+SingleMainWindow=true