NLA: Assign Controls to Strips

This goes hand in hand with blender.community/c/rightclickselect/VHfbbc/ (NLA: Extract Animations for Control), to prevent accidental layer inclusion when keyframing. NLA strips would support assigning controls. If the user attempts to key a control not assigned to the active strip then the operation fails and shows a visual warning. Strips with no assigned controls should accept all keyframes.