Improve handling of multiple/no webcams.

- If no webcam is found, display a message that no webcam was found.
- Fix visibility of webcam chooser combo box if two webcams are plugged in.
- Handle removal of webcams, automatically switch to another webcam.
Previously, if you had two webcams and unplugged the current webcam,
you could not switch to the other one because the chooser combo was hidden.

REVIEW: 112751