Add playback position seek function

[pushback]

You can only play from the beginning of a song.
Add playback position seek function.

Implementation method:

• Get total time and current position from "mpc" result
• Display seek bar from total time and current position (use <input type = "range"> or UI liblary)
• Get position with onchange event and calculate time of moving position
• Execute "mpc seek HH: MM: SS" using the calculated time