Fix MPRIS seeking and SetPosition

[ferdnyc]

Correct the math we use to perform seek operations via MPRIS, since KDEConnect uses microseconds for seek offsets even though they use milliseconds for position values. (MPRIS uses microseconds everywhere.)

Also, implement SetPosition() directly instead of computing the offset from the current position and calling Seek(), because some players don't respect arbitrary Seek() lengths.

Picking up the currently-playing trackid at the time the SetPosition() call is handled and assuming it's the same track it was intended for is somewhat risky, but picking up the current Position and making the same assumption (which is what the previous code did) carried the exact same amount of risk.

Fixes: #1779

[andyholmes]

LGTM, nice investigation on Chrome weirdness and bonus points for the comment.

[ferdnyc]

Might as well get this one in too, before we start making release noises.