development_io (quick_link, expander... socket props and support in IO/JSON)

Created by: zeffii

Things that are still issues before hitting master

• socket custom props are stored, but isn't sparse.
• nodule_color (but only used in color-nodes for a limited period from this point...and gp node
• restore custom_draw
• correctly store use_custom_color

condensed changelog.

• Return zero vector for no input in VerticesSocket
• Add Sv prefix to Color and Quaternion socket classes
• Add "link matrix" UI/operator for MatrixSocket
• Add "link vector" UI/operator for VerticesSocket
• Add quaternion to matrix socket conversion
• Update collapse expander for color to use SvColorSocket name
• Add matrix to quaternion socket conversion *Add draw_link_new_node to socket base (common) to allow various inputs to add button to spawn and link different input nodes.
• Minor class/idname changes to the link new node
• Add "use quicklink" flag (To allow the quick link feature to be turned on/off if needed.)
• Socket specific draw calls were replaced by the draw call in the base class.
• adjust the quicklink new node offset to avoid overlapping new nodes (new nodes are now offset based on their index).
• Add code to handle export/import of socket custom props
• Optimization to draw_expander_template