Mpyg321

- Adds `toggle_play` and `toggle_pause` functions

Adds support for events. You can now write callbacks for events by using the new decorators or the `subscribe_event` function:

player = MPyg123Player()

player.on(MPyg321Events.ANY_STOP)
def callback(context):
print("Any stop event occured")

or
def my_func(context):
print("Other event subscribed")

player.subscribe_event(MPyg321Events.ANY_STOP, my_func)

Resolves small bug that would crash the wrapper.

Implements custom args + rva-mix option.
Removes useless legacy code.

Fixes bug when using audiodevice.

- We now have 2 separate classes for mpg123 and mpg321
- This allows the players to only contain the functions that they support

Breaking changes:
- mute and unmute are only available for mpg123
- MPyg321Player only supports mpg321. People using mpg123 should migrate to `MPyg123Player` which has more features for mpg123.