add boilerplate for custom defaults

Created by: zeffii

this PR allows us to write a node_defaults.json, with two forms of directive

1. props
2. function

it will be extended to add

3. sockets_states , when socket.set_state() is implemented.

datafiles/sverchok/node_defaults/node_defaults.json

{
    "SvGenFloatRange": {"props": [["stop_", 1.0], ["count_", 20]]},
    "ViewerNode2": {"props": [], "function": "view3d.init_vdmk2"}
}

• it's possible to push props in sequence with a list of prop_name, prop_value pairs.
• it's possible to push a function, in the example above the function is referenced as view3d.init_vdmk2. read that as filename.function. and filename shall be located in datafiles/sverchok/node_defaults/.

datafiles/sverchok/node_defaults/view3d.py

import random
import colorsys

import bpy



def init_vdmk2(node):

    def fold_color(color, shift):
        h, s, v = colorsys.rgb_to_hsv(*color)
        return colorsys.hsv_to_rgb(shift, s, v)

    A = [0.938000, 0.948000, 0.900000, 1.000000]
    B = [0.500000, 0.752000, 0.899000, 1.000000]
    C = [0.030100, 0.488000, 0.899000, 1.000000]

    shift = random.random()
    node.vertex_colors = fold_color(A[:3], shift)
    node.edge_colors = fold_color(B[:3], shift)
    node.face_colors = fold_color(C[:3], shift)

    node.edge_width = 1.0
    node.vertex_size = 2.0

---

Each time you add a node, the node_default_dict is checked if the bl_idname has an entry.

• this has no effect on nodes that you duplicate, they clone their properties according to self.copy()
• this doesn't offer a visual way to set the preferences yet, that would be the next step.