Making seeker easier to use

[waveyboym]

Changelog

1. Adding volume parameter to SharedAudioManager to keep track of the audio volume level as kira does not have a get_volume() function.
2. Added seek_by function exposed to the frontend. This function will allow seeking by a specified value, so if the value is 2secs, it will seek the playing position of a song 2secs ahead.
3. Added a delay to seeking using the handle_true_seeking function. There are comments there that explain further the issues and the reasoning for why that function exists. The function can be found in player.rs