The big background cleanup

Given that @exalm is working on making the shell switch to a separate wallpaper in dark mode, and me planning to pick up the transparent panel plans again, we'd like to clean up the amount of background modes gnome-shell supports.

That involves getting rid of a bunch of features:

• Solid color backgrounds, i.e. primary-color and secondary-color keys of org.gnome.desktop.background
• Color shading, i.e. color-shading-type key of org.gnome.desktop.background
• Animated wallpapers, the dark mode wallpaper will somewhat replace that
• Wallpaper style options, basically all options except "Zoom"
• Support for wallpapers with alpha channels

Each of those features except animated wallpapers has not been configurable in g-c-c for quite some time, and even Tweaks doesn't allow setting solid colors or shading.

This will not only make mutters and gnome-shells background code significantly more maintainable, it should also allow removing all the GnomeBG code from gnome-desktop.

To say it in another way, that would basically only leave fixed image wallpapers rendered using the "Zoom" method.

cc @bertob @aday @jimmac