KDE Plasma's wallpaper picker looks for wallpapers under /usr/share/wallpapers/
with a metadata.desktop file, not just /usr/share/backgrounds/. Added install
target to /usr/share/wallpapers/antergos-wallpaper/contents/images/ and
included metadata.desktop so the antergos wallpaper appears automatically in
System Settings > Wallpaper instead of requiring manual browsing.
Bumped pkgrel 4->5 to trigger rebuild.
