Opencv-proto

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+ Added 'Legend' into primitives subpackage.
+ Added .describe_properties() helper to adjust Video/Camera props with trackbars.
+ Added Frame.draw_rectangle().
+ Added more color names.
+ Exposed more adjustable properties for Video and Camera.
* Now can be installed with the thirdparties: pip install opencv-proto[all].
* Text.apply* renamed into .put_on* for clarity.

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+ Added Frame abstraction.
+ Added helpers to save images.
* Video.write() renamed into .dump() as less confusing.

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+ Basic functionality.