deck: background and actions optional slots

[peterpeterparker]

<deckgo-deck>
  <!-- X slides deckgo-slide-whatever -->

  <something slot="actions"></something>
  <something slot="background"></something>
</deckgo-deck>

Furthermore than the attributes the deck could contains two slots background and actions. We should persist these in the database in the entity deck. Type is a "blob" (identical to slide.content).

P.S.: I need this for the stock photos