New attribute "is_group" for light groups

Component/platform: light groups doc

Description of problem: impossible to differentiate between actual lights, and light groups.

Relevant configuration.yaml entries:

- platform: group
  name: rooma
    - light.bulb1
    - light.bulb2

Description: There are scenarios (like this) where it would be helpful to be able to differentiate between actual lights (light.bulb1 and light.bulb2 in the configuration above) and a light group (eg. light.rooma).

Suggestion: This could be done by automatically adding an attribute to light groups; for instance is_group = true, similar to the attribute is_hue_group: true which is set for light groups imported through hue.