js/playList: parse rating to float

the custom sort function is comparing strings. String comparision in JavaScript is fine as long as both strings have the same lenght and the decimal mark at the same position.

But "10.0" was the only case violating this. "10.0" < a is returning false for all a's matching ^[0-9]\.[0-9]$

fixes #374

v2: we need to handle the rating coloumn explicitly, also remove this dirty eval hack with an associative array access

fixup
Merge pull request #541 from karolherbst/bug374

js/playList: parse rating to float