Rework background color & image handling

[Degubi]

• Rename existing 'background' property to 'backgroundColor'
• Add new setting 'backgroundImage' with values:
  • 'none' means disabled (backgroundColor gets used)
  • some other cube images that could be found in the textures/cube directory
• Fix some issues regarding background color changing. Previously when changing the background color it would disappear and the body element's green background would show up.
• Remove green background flash while loading in
• Re-organize functions in viewer.js, so that all user input handling functions are at the same place