UI: Use instanced panel custom data instead of list index

For modifier shortcuts we added a "custom_data" field to panels.
This commit uses the same system for accessing the list data that
corresponds to each panel. This way the context is only used once
and the modifier for each panel can be accessed more easily later.

This ends up being mostly a cleanup commit with a few small changes
in interface_panel.c. The large changes in the UI functions are due
to the fact that the panel custom data is now passed around as a
single pointer instead of being created again for every panel.

The list_index variable in Panel.runtime is removed as it's now
unnecessary.

Differential Revision: https://developer.blender.org/D8559

release/scripts/startup/bl_ui/properties_constraint.py

@@ -114,11 +114,7 @@ class ConstraintButtonsPanel(Panel):
                 col.prop_search(con, "subtarget", con.target, "vertex_groups", text="Vertex Group")
 
     def get_constraint(self, context):
-        con = None
-        if context.pose_bone:
-            con = context.pose_bone.constraints[self.list_panel_index]
-        else:
-            con = context.object.constraints[self.list_panel_index]
+        con = self.custom_data
         self.layout.context_pointer_set("constraint", con)
         return con
 
@@ -963,11 +959,7 @@ class ConstraintButtonsSubPanel(Panel):
     bl_options = {'DRAW_BOX'}
 
     def get_constraint(self, context):
-        con = None
-        if context.pose_bone:
-            con = context.pose_bone.constraints[self.list_panel_index]
-        else:
-            con = context.object.constraints[self.list_panel_index]
+        con = self.custom_data
         self.layout.context_pointer_set("constraint", con)
         return con