fix: remove (imprecise) seconds->milliseconds conversion

[f0e]

The new millisecond to second conversion code has an issue, the cast to int removes float precision. For example if you pass 1.5 seconds in it'll be turned into 1 second, then 1000 milliseconds. This caused my music progress duration to go up and down a second every time I updated it.

Rather than fix this, I think it'd be better to revert the start and end arguments to be milliseconds again as changing them to seconds is a breaking change that I don't personally see being a benefit.

This PR:

• Makes start and end arguments (integer) milliseconds again
• Updates the docs to explicitly state that these arguments are milliseconds
• Removes the test case for millisecond to second conversion

[kaneryu]

Thanks for the work!