Duration slider: update music position when seek slider is released

Copied implementation from Koko

• The music player now updates its position when the duration slider handle is released.
• The music continues playing as normal and doesn't jump around whilst the slider is being moved.
• The slider no longer carries on moving while the handle is being held.
• There is now a tooltip on the slider handle that shows the slider's position (only shows when the handle being held)

Current bug (also in Koko):

• The tooltip doesn't actually show above the slider handle (it shows to the top-left of the slider itself) -- not sure why