Several minor code improvements.
Actually, tracks.js contains several pieces of code which could
be re-written more effectively. The problem is that in some places
there are conditional checks like this:

    if (condition) {
        call_setter( true);
    } else {
        call_setter( false);
    }

which could be re-written to:

    call_setter( condition);

This improvement saves some bytes of code length, traffic and should
also work more effectively by utilizing less machine resources