Major refactor and simplification of cast.py

[xsdg]

• Resolves ambiguity of available_devices as being a method or a list
• Creates an AvailableDevice dataclass to simplify data access
• Moves print_available_devices helper to cast.py
• Creates a currently-non-functional backup and drops Sonos support from main Casting class
• Drops long-deprecated reboot support

Also, this avoids using available_devices as a global variable, which makes progress on #425