Add keys and __contains__ method to Controls class

[nhallez]

The Control class (video) inherits from dict but only overrides items() and values(), leaving keys() and the in keyword to fall back to the default dict implementation. This causes those methods to return an empty result immediately after device creation, before any refresh has occurred.

As a result, membership checks like:

ControlsID.FOCUS_AUTO in cam.controls

…silently fail unless controls.refresh() is explicitly called.

[tiagocoutinho]

Looks good!
Thanks for the contribution
Merging