postmarketos-artwork: add wallpapers/icons/sounds subpackages

Since we never used this "meta package" to install the other packages, I suggest to just use it to build various subpackages with all the other artworks.

I've added the icons and the sounds subpackages, and moved the wallpapers/wallpapers-extra here (from the removed postmarketos-artwork-wallpapers aport)

Also included some minor improvements:

• Updated to last commit
• Added the other postmarketos wallpapers to the wallpapers-extra subpackage
• Install the files with proper permissions (644)