Appearance panel, part 1

I left out for now:

• Screen transition (gnome-shell!2070 (merged)- g-c-c counterpart is a tiny commit on top of this so not including yet)
• Dark wallpapers
• Scheduling

Depends on adwaita-icon-theme#154 (closed)

Shell counterpart: gnome-shell!2069 (merged)

#1523 (closed)