Eagle Double Headed needs `legged` property

As of now, the charge Eagle Double Headed can only be coloured specifically on its talons, beaks, and tongues (`armed`, `beaked`, and `langued` respectively), apart from its body of course.
However, its legs cannot be coloured differently than its body, unlike with Eagle Displayed. On many coats of arms, this part of the bird is coloured differently to its body.

未指定文件或测试。首先定位 Eagle Double Headed renderer，并将其与已经支持使用不同颜色显示腿的 Eagle Displayed 进行比较。完成标准是：`legged` 属性可以独立地为腿着色，同时保留现有的 `armed`、`beaked` 和 `langued` 行为。

由索引模型根据 Issue 内容生成。