tenacity

Part 0: Fundamentals

Introduce class AudioBackend, an abstract class that provides a common interface around implementing different audio backends.
- Introduce PortAudioBackend, an implementation of this class using PortAudio.
- AudioBackend instances handle streams and devices.
- This class has the following member types:
  - NativeStreamType - The native type for streams (e.g., PaStream for PortAudio)
  - NativeDeviceType - The native type for device handles (e.g., int for PortAudio)
Introduce AudioBackendManager, which controls which backend implementation is active.
- AudioBackendManager owns a shared_ptr to an AudioBackend instance. You can retrieve a pointer to this instance when needed.
- Only one backend may be active at a time. Backends must free all their resources on destruction (RAII).
Maybe introduce some new exceptions?
- GeneralException - Something happened. All exceptions listed below are derived from this type
- `UninitializedBackend - The backend was not initialized.
- BadDevice - The device handle was bad.
- StreamError - An error opening a stream occured
Maybe separate device management functionalities from AudioBackend into its own class?

Part 1: Changes in DeviceManager

Move all PortAudio device management functions into PortAudioBackend.
Make DeviceManager cooperate with AudioBackendManager to get a weak_ptr to the audio backend.