Unify camera providers

[falkoschindler]

This PR tries to unify different camera provider classes:

• The active flag has a different meaning. We will rename it to is_connected and let it represent the device state, not whether the user wants it to be active.
• Maybe we'll introduce enable()/disable() methods and an is_enabled property to allow the user to enable/disable the capturing process.
• MultiCameraProvider and CombiningCameraProvider are doing the same. We can remove the latter.
• The CAMERA_ADDED event should be emitted more consistently.

[falkoschindler]

We came up with the following class hierarchy for structuring cameras and camera providers in our projects:

RoSys

CameraProvider
UsbCameraProvider
RtspCameraProvider
SimulatedCameraProvider

CalibratedCamera [mixin]
CroppableCamera [mixin]
ExposureCamera [mixin]

Camera
UsbCamera(CroppableCamera) -> UsbDevice
RtspCamera(CroppableCamera) -> RtspDevice
SimulatedCamera(CroppableCamera)

Project 1

FolderCameraProvider
FolderCamera

GalaxyCameraProvider

CameraScanner
IpCamera

Project 2

WifiCamera(CalibratedCamera, Camera)
WifiCameraProvider
WifiCameraProviderHardware
WifiCameraProviderSimulation
WifiCam

[NiklasNeugebauer]

Upon further review and if I understand asyncio correctly, the locking mechanism will not be necessary. In fact, the old problem should also be fixable without a lock by moving the check for the existence of the device inside set_parameters.