rewrite input-demoseeking.cfg

rewrite input-demoseeking.cfg

- Fix "defer clear" commands breaking seekdemo.
- Fix repeated seekdemo commands often breaking the seek.
- Handle failed playdemo command cleanly.
- Introduce maximum playback speed to avoid freezes for long seeks.
- Introduce option to disable default bindings.
- Introduce hooks for demo playback start and seek end.
- Introduce alias for retrieving seek target or demo playback time from
  scripts.
- Introduce vars to provide information about demo playback.
- Prevent interrupting the game and loading the last viewed demo when
  accidentally running seekdemo command during an actual game
  (cl_hook_gamestart_all setup needed).
- Reduce screen flashes by disabling r_render only for long seeks where
  the performance is needed.
- Disable more options to improve performance and reduce particles and
  effects accumulating during the seek.