Define default colour scheme for nodes and add preferences mechanism

[bentoogood]

• We need to design a suitable default colour scheme for node types.
• Then given that users may personal preferences over colour schemes, we should provide a preferences mechanism for user to set their owns schemes (still driven by type)
• Additionally, users may wish to manually colour specific nodes in their graphs. This will require a new per node colour mechanism.

[bentoogood]

being handled graffer side for now, so moving out of milestone.

[bentoogood]

A detail to consider when implementing the node colouring gaffer side.

In graffer right now, if I duplicate a node (ctrl-c,ctrl-v) the duplicate does not inherit any nodeColor that the source has.

[johnhaddon]

The reason this is happening in Graffer is that the Graffer nodeGadgetCreator decides on a colour based on the shader being held on the node. But when the node is first added to the script during cut and paste, the shader has not yet been loaded.

[johnhaddon]

This relates to #89, so they may as well be done together.