Rework the v4l2 control handling. Variables corresponding to controls don't...

Rework the v4l2 control handling. Variables corresponding to controls don't have any v4l2 suffix anymore. Variables for custom controls are also created. See "vars v4l2" in the rc interface. The "controls" variables is a list of all controls available. Next step is to use all these changes in the Qt interface to create a v4l2 control pannel.