Graph Editor : Allow background color to be specified

## Issue

The background color for the node graph area is hard-coded. There is the desire from users to be able
to control this.

## Proposal

Expose this as a command line argument: 
```
--graphBackground <R,G,B>
```
where <R,G,B> is a color tuple in the range 0..1 for each channel.

When not specified the current default color is used.
