Implemented automatic directory creation for Render displays.
Added automatic rib directory creation for RenderManRender node.
Added automatic ass directory creation for ArnoldRender node.
Defined standard project variables and used them to define default display, rib and ass locations.

This is all just defined in the default startup files for the gui app. Other applications are free to define totally different conventions or no conventions at all. Startup files with precedence in the searchpath can change the values for the project variables, remove them, or define variables of their own.

Fixes #407 and #59.
Added GraphComponent::clearChildren() method.

It seems worth having a specific method for this, rather than forcing everyone to roll their own, particularly because it can be much faster to delete from the end backwards, which might not be obvious.
Fixed ScriptNode::load() to clear existing variables.

This fixes a problem with File->RevertToSaved.
Greyed out File->RevertToSaved menu item when it doesn't make sense.