View Layers currently cannot be reordered, forcing Python programmers
to remove and recreate them if they want to sort them.
This commit adds a new method `Scene.view_layers.move()`. It acts the
same as `Object.modifiers.move()`, taking two arguments `from_index`
and `to_index`.
Layers could then be reordered like this, for example:
``` python
import bpy
scene = bpy.context.scene
layer_names = scene.view_layers.keys()
sorted_layer_names = sorted(layer_names)
for l_i, l in enumerate(sorted_layer_names):
scene.view_layers.move(scene.view_layers.keys().index(l), l_i)
```
Damien Picard