fix: Wait for seek to complete

[Gustl22]

Description

Wait for seek to finish by listening to the AudioEventType.seekComplete event.

• need to validate if the tests are passing multiple times after each other
• test what the Playbin GstState is after seeking

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, refactor:,
  docs:, chore:, test:, ci: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in example.

Related Issues

[Gustl22]

Is it ok to merge? I added quite a lot commits since your review @spydon :)

[spydon]

Lgtm, merge awaaay! 😄

[japostoles]

If the AudioPlayer is in a stopped state, the seek function will never complete, and the TimeoutException will fire 30 seconds later. ([ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: TimeoutException after 0:00:30.000000: Future not completed)

Any code after the seek will not be executed. From the perspective of a user of the library, the application will hang 30 seconds before registering an unhandled exception.

Sample code:

    var player = AudioPlayer();
    await player.setSource(DeviceFileSource(SomeAudioFile));
    await player.resume();
    sleep(Duration(seconds:1));
    await player.stop();
    print('-SEEK-');
    await player.seek(Duration(seconds:15));      // --- HANGS FOR DURATION OF TIMEOUT HERE
    print('-SEEK FINISH-');