Add `eqN_keytrack` & `eqN_keycenter`

[pgcrowe]

While SFZ has fil_keytrack for filters, it would be extremely useful to provide the same keytrack functionality for EQ. I humbly request implementation of a new eqN_keytrack=cents opcode or similar, that would increase the value of eqN_freq, the same way that fil_keytrack does for cutoff\cutoff2.

To match behavior of the other keytrack implementations, it would make since to likewise implement a eqN_keycenter opcode, similar to fil_keycenter / fil2_keycenter.

I came upon the idea for eqN_keytrack while experimenting with implementation of tremolo/vibrato. With real wind instruments, each overtone varies slightly in amplitude and pitch depth. While I imagine that implementing pitch modulation for individual overtones would be a challenge (likely require a vocoder and intensive processing power), Sfizz's existing eqN filters and randomization do a beautiful job at modulating the pitch of individual overtones. However, at present, the only way to achieve a tremolo that works on individual overtones, is to write out filters for every single sample, which is extremely clunky and time consuming. eqN_keytrack would make much easier work of this.

Thanks!
Pat