OpenMW-CS: Improve scene view colors

Hello,

I have a small-ish proposal for the scene view in CS. By default, the background is purple and there are orange widgets for displaying neighbouring cells. The colours are not exactly neutral, nor do they help with the editor's aesthetics.

As the simplest change, the background could be changed to a shade of grey, with the arrows made blue.

With some more change, the background could also be a gradient, with colours defined in the user preferences.

Pushing this even further, the gradient could optionally depend on which light is active in the scene view. When the sun is shining, the gradient is brighter, and when the lighting is set to night, the background gradient is darker.