Port particle instance modifier changes from Gooseberry branch

The work is mainly from Lukas Toenne, with some modifications from myself.

Includes following obvious changes:

- Particle system selection is now name-based, with lookup menu.
- Lots of new options to control varieties.

Changes comparing to the Gooseberry branch:

- Default values and versioning code ensures same behavior as the
  old modifier.

- Custom data layers are coming from vertex color, the modifier
  does not create arbitrary layers now. The hope is to keep data
  more manageable, and maybe make it easier to select in the shader
  later on.

  This means, values are quantized to 256 values, but it should be
  enough to get varieties in practice.

Reviewers: brecht, campbellbarton

Reviewed By: brecht

Subscribers: eyecandy

Differential Revision: https://developer.blender.org/D3157

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -711,11 +711,15 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
 
     def PARTICLE_INSTANCE(self, layout, ob, md):
         layout.prop(md, "object")
-        layout.prop(md, "particle_system_index", text="Particle System")
+        if md.object:
+            layout.prop_search(md, "particle_system", md.object, "particle_systems", text="Particle System")
+        else:
+            layout.prop(md, "particle_system_index", text="Particle System")
 
         split = layout.split()
         col = split.column()
         col.label(text="Create From:")
+        layout.prop(md, "space", text="")
         col.prop(md, "use_normal")
         col.prop(md, "use_children")
         col.prop(md, "use_size")
@@ -726,6 +730,10 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
         col.prop(md, "show_unborn")
         col.prop(md, "show_dead")
 
+        row = layout.row(align=True)
+        row.prop(md, "particle_amount", text="Amount")
+        row.prop(md, "particle_offset", text="Offset")
+
         layout.separator()
 
         layout.prop(md, "use_path", text="Create Along Paths")
@@ -737,8 +745,16 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
         col.prop(md, "use_preserve_shape")
 
         col = split.column()
-        col.prop(md, "position", slider=True)
-        col.prop(md, "random_position", text="Random", slider=True)
+        col2 = col.column(align=True)
+        col2.prop(md, "position", slider=True)
+        col2.prop(md, "random_position", text="Random", slider=True)
+        col2 = col.column(align=True)
+        col2.prop(md, "rotation", slider=True)
+        col2.prop(md, "random_rotation", text="Random", slider=True)
+
+        col = layout.column()
+        col.prop_search(md, "index_layer_name", ob.data, "vertex_colors", text="Index Layer")
+        col.prop_search(md, "value_layer_name", ob.data, "vertex_colors", text="Value Layer")
 
     def PARTICLE_SYSTEM(self, layout, ob, md):
         layout.label(text="Settings can be found inside the Particle context")