Breaking changes in main branch

[Makuna]

The next release will include a few changes that will break sketches written for previous versions. The progress of this work will be in the main branch.

If you pick up this library from Arduino Library Manager, then don't worry, it's not released yet. But it will be at some point.
If you pick up this library from Github, then make sure you pick up from the last released tag version v1.2.3 to avoid getting these changes.

How notification callbacks are made will change.

• support the ability to not implement nor get notification callbacks through a notification object
• support the ability to have callbacks made directly to your object through non-static methods.
• removed the mp3 object pointer from the notification method callbacks.