Customize panels should be unique for each widget type. Further
uniqueness should be achieved by controlling which attributes
and elements are customizable.
All customize panels should be added to the Customize Menu
sidebar (Except the elements of the preview window).
Control widgets should be connected to the object's attribute
that it controls.
Set certain attributes as "primary" customizable attributes,
which the panel will show by default. Other attributes will be
controllable after showing "Show all"
To support the background swapping, you will need to put most
layouts into their own widget