Rescale internal Slider values to avoid integer overflow

The original implementation used the raw duration and position as
reported by qmultimedia, which are in milliseconds. This, combined with
the actual Slider implementation in qqc2-desktop-style caused
value/duration to overflow at about 3 minutes and 30 seconds.
The adapted implementation uses seconds for the internal Slider values.

CCBUG: 455339


(cherry picked from commit 98227f05)