Add an API for camera access

[freakboy3742]

What is the problem or limitation you are having?

Most laptops and phones have one or more cameras attached. Toga should have a mechanism to capture still images and videos from those devices.

Describe the solution you'd like

Something like the following API should be possible:

if myapp.camera.has_permission:
    image = myapp.camera.take_photo()

Describe alternatives you've considered

Use an existing camera library, rather than writing a new one.

Unfortunately, the state of the art isn't very promising for re-use:

• OpenCV convolve "camera access" with "performing analysis and transformation of the image". It also has its own windowing/UI layer, which would need to be integrated with Toga; and it has dependencies on scikit and numpy, which is a heavyweight requirement for a simple "just take a photo" app.
• SimpleCV is a lightweight wrapper around CV2, but has many of the same problems.
• Kivy has APIs for camera access, but they're tightly integrated into the Kivy
• GStreamer might be usable as a base implementation on GTK, but won't be easily usable on other platforms.
• PiCamera is specific to Raspberry Pi camera hardware
• webcam is a wrapper around cv2
• Pygame provides a camera API (with multiple backends), but it's not usable as a standalone package. The builtin backends might be useful as inspiration for Windows/Linux camera handling.

Additional context

There's also the issue of permissions to consider, which will likely need changes to Briefcase (see beeware/briefcase#547).