Implement custom names for render slots

Basically the title tells it all, quite straightforward implementation.

The only thing is the image.render_slot which used to represent the active
render slot index is now moved to image.render_slots.active_index.

Reviewers: venomgfx, campbellbarton

Differential Revision: https://developer.blender.org/D821

release/scripts/startup/bl_ui/space_image.py

@@ -661,6 +661,11 @@ class IMAGE_PT_view_properties(Panel):
             sub.active = uvedit.show_stretch
             sub.row().prop(uvedit, "draw_stretch_type", expand=True)
 
+        if ima:
+            layout.separator()
+            render_slot = ima.render_slots.active
+            layout.prop(render_slot, "name", text="Slot Name")
+
 
 class IMAGE_PT_tools_transform_uvs(Panel, UVToolsPanel):
     bl_label = "Transform"