Refactor ‹dia-controller› to allow opt-in/out of child controllers

[olange]

Keyboard and remote Firebase controllers are now built-in the Shadow DOM of the ‹dia-controller›. The user cannot opt-out from Firebase remote controller, for instance.

Refactor those elements and their classes, to allow the user to control which controller she·he wants to enable in his slideshow. They should not come automatically enabled, when installing the packages.

Actions

• Clarify communication between DiaKeyboardController and DiaController DONE 22.03 actions via custom events now, instead of method calls
• Y TODO
• Z TODO

See also

• Keyboard controls to navigate the slideshow #5 Keyboard controls to navigate the slideshow
• Remote control to navigate the slideshow #6 Remote control to navigate the slideshow