Add overloaded typing information to `MPDNode.attr()` to be able to
have more precise typing of return values. Return values of required
attributes are still typed as optional and need to be overridden.

Also change the order of parameters.

- Fix order of `*args` and keywords
- Remove `*args` from `MPDNode`
- Capture `node`, `root` and `parent` in `*args` and `**kwargs`
- TODO/PEP570: set positional-only and keyword-only parameters (py38)

- Add typing annotations to all `MPDNode` children
- Fix various typing issues
  - Override optional types of required attributes
  - Add checks for optional stuff in segment processing
  - Fix processing of empty node texts
  - Fix typing in `DASHStream` caused by enabling typing in `MPDNode`s