Resolve "Pre-Comp scene manager"

• create 2 write nodes for groups, 1 for line art and 1 for colour
• link write nodes
• position write nodes at the bottom of the node view in the backdrop
• create backdrop if its missing (optional)
• disable all cameras (optional)
• create a Precomp_Camera-P
• connect the peg to the camera and any parent peg if there is one
• disconnect the 'Camera-P', unlink it and disable it

Closes #44