Release v21.0

New Changes:

• Reimplement UI using pages
• Added a new 'About' page, with support for GNOME 42
• Added a new 'Credits' page, for extension developers and translators
• Replaced preferences title with page switcher
• Replaced extension icon with an svg
• Miscellaneous settings menu design improvements
• Updated README and showcase screenshots
• General code fixes (Primarily target GNOME 40+, styling, code quality improvements)
• Build system improvements (Output, structure)