Initial encoder layout support

[cvpcs]

Adds a new option to use encoder layouts on dials for Stream Deck Plus devices:

When enabled, entities will be displayed using one of two layouts:

• Indicator Layout
  This will be used to display the volume level on media_player entities, or the brightness on light entities that support brightness levels:
  

• Value Layout
  This will be used to display all entities that don't otherwise have support for another specialized layout. The value will be set to the entity's state value:
  

[cgiesche]

Hi and thank you very much for your great work! I try to review and test this as soon as I can.

I am currently working on a complete overhaul of the way the icons are generated. I hope I can integrate your changes there painlessly :)

https://github.com/cgiesche/streamdeck-homeassistant/tree/feature/254_Improve_display_image_rendering

[cgiesche]

Spoiler:

I merged your changes with mine and after some brainf***, is looks promising.