Rigify: annotate and fix warnings in basic rig components.

Introduce a method to annotate types and names of entries in the
`bones` container of rig components and apply it, and other type
annotations, to a number of not very complex rig classes.

- Introduce BaseRigMixin as a typed base class for mixins intended
  for use in rig classes (using BaseRig as a parent causes issues).
- Introduce TypedBoneDict that does not suppress the unknown attribute
  analysis in PyCharm, and use it in a system of subclasses to
  annotate the bones in various rigs. BaseBoneDict is necessary
  because the annotation affects all subclasses, so TypedBoneDict
  cannot inherit from BoneDict with the annotation.
- Add or adjust other type annotations of rig methods and utilities.
- Fix other warnings, e.g. undeclared attributes, excessively long
  lines, whitespace style issues and typos.