[Camera] Add method to switch to a specific video device

[YuAo]

See the discussion in #37 (comment)

[dsmurfin]

This looks great, thank you. I will look at adding the equivalent for audio input switching.

[YuAo]

@dsmurfin For the audio part:

It behaves differently when disabling video/audio output with the current implementation. When disabling a video output, video input is kept, however disabling an audio output removes the audio input.

Since no audio device selection is allowed previously, this is not a big problem - we always use the one default audio device. But now, I think it's better to unify this behavior.

I suggest that we keep the audio and video inputs when disabling the outputs, and provide methods to remove the inputs (as you suggest in #36):

• This avoids the need for reconfiguring the device after disabling the output.
• It makes the enable...output/disable...output methods work more like their name suggests (except that the enable...output methods also config the input device when necessary, which I think is a nice helper).