Refactor `walk_back()`:
- compare ancestor nodes by type and not by tag name
- rename mapper parameter and check its result before yielding the node

Refactor `walk_back_get_attr()`:
- allow finding attributes from specific ancestor nodes only
- stop iterating after the first match

Refactor `attr(inherited=...)`:
- allow inheriting from ancestor nodes, optionally from a specific type,
  and not just the parent node

- Make `AdaptationSet` and `Representation` inherit from the common
  `_RepresentationBaseType` class
- Move and logically group class definitions
- Define common attributes in `_RepresentationBaseType` that get
  inherited from ancestor nodes of the same base type, and find
  common child nodes
- Fix, clean up, reorder and selectively add attributes
  - Remove `Representation.numChannels` (invalid and unused)
  - Turn `Representation.lang` into a property which reads its value
    from the parent `AdaptationSet.lang`
    (it's not an attribute on `Representation` according to the spec,
    but it gets read by `DASHStream.parse_manifest`, so keep the alias)
  - Rename `contentProtection` attribute to `contentProtections`
    and `subRepresentation` to `subRepresentations`, to stay consistent
    with other attributes of child node lists
- Fix tests with old assertions: non-inhertied attributes that led to
  different expectations (e.g. the resulting stream name)

- Make `SegmentBase`, `SegmentList` and `SegmentTemplate` inherit from
  the common `_SegmentBaseType` and `_MultipleSegmentBaseType` classes
- Logically move class definitions
- Define common attributes in `_{,Multiple}SegmentBaseType`
  that get inherited from nodes of the same type in ancestor nodes,
  and find common child nodes
- Find inherited attributes independently
- Fix, clean up, reorder and selectively add attributes
  - Add `_SegmentBaseType.availabilityTimeOffset` (currently unused)
- Fix `MPD.minBufferTime` type (required attribute which can't be None)