Add pitchControl to indicate if a voice can be pitch adjusted

[HadrienGardeur]

As identified by @Falcosc in #34, some voices do not allow users to change their pitch.

With pitchControl, I'm adding a new boolean that will default to true but allow us to identify voices where this feature is unavailable.

As I mentioned in #34:

Using this new boolean, implementers could disable pitch control accordingly. It's better to disable pitch (or grey it out) than display a component that won't change anything for the user.