Revisit playback states

[mat-hek]

We should revisit the shape of playback states, considering the following:

• [ ] Playback states currently are not really related to the actual playbacks, but rather the readiness of elements to process the stream
• [ ] Children shouldn't be spawned in handle_init, but in handle_stopped_to_prepared, when other resources are allocated
• [ ] Going between stopped and playing multiple times was never used and many elements don't support that. We should remove that possibility if it's not needed, possibly renaming handle_stopped_to_prepared to handle_continue
• [ ] Determine when and if playback states should be synchronized
• [ ] Consider simplifying suspending playback state change (by creating more playback states instead?)
• [ ] Make playback handler not use callbacks to reduce code spaghettiness

Related to #55 and #64