Add auto accent color based on wallpaper

[lenemter]

Fixes #569

[Marukesu]

So, i think a better way to do this is to have the SettingsDaemon provide the accent color (in the same format as the portal, instead of a enum with the selected accent), then Greeter and Gala would only need to get the approximate theme. (in fact, there's elementary/gala#1674 about using the portal for the theme accent there).