这是一个面板，自定义程序包含三个层：panels 包含节，sections 包含设置，以及settings 是由controls 在customizer UI中。
[panel]
|--[section]
| |--[setting] and his [control]
| |--[setting] and his [control]
|--[section]
| |--[setting] and his [control]
| |--[setting] and his [control]
Panel,
Section,
Setting,
Control创建Panel 在创建Sections 通过$id 将它们分组在同一个Panel.
