add resources

add image resources:

• cables
• external connection
• nodes
  • fallback for unknown cpu
  • fallback for known cpu with unknown configuration
  • fallback for known cpu + size with unknown configuration
  • specific to cpu + configuration
• empty tile (the tile class empty tile not null)
• board background
  • fallback for unknown size

all resources also need a special variant for unremovable/unmovable tiles.
all resources also need a special variant for immutable tiles.
node resources for the editor are seperate from the other node resources so that an immutable editor/port-configuration is possible.