added onclick-handler to heading

[wehrstedt]

This PR adds an onclick-handler to the heading section.

Configuration example:

type: 'custom:banner-card'
heading: "\U0001F6CB Wohnzimmer"
heading_entity:
  entity: switch.wohnzimmer
entities:
  - entity: light.wohnzimmer
    name: Licht
  - entity: cover.rollo_wohnzimmer_garten
    name: Rollo
  - entity: sensor.wohnzimmer_temperatursensor

Output when switch is active:

Output when switch is not active:

If you click on Wohnzimmer (btw Living Room), the switch will be toggled. But you can override the action as described in the readme.