UI: Add "Remove All Materials" operator

Added a new operator `OBJECT_OT_material_slot_remove_all`
that removes all materials from the material slots of selected objects

This was inspired by a request proposal on RCS.

Pull Request: https://projects.blender.org/blender/blender/pulls/138402

scripts/startup/bl_ui/properties_material.py

@@ -21,6 +21,7 @@ class MATERIAL_MT_context_menu(Menu):
         layout.operator("object.material_slot_copy")
         layout.operator("material.paste", icon='PASTEDOWN')
         layout.operator("object.material_slot_remove_unused")
+        layout.operator("object.material_slot_remove_all")
 
 
 class MATERIAL_UL_matslots(UIList):

scripts/startup/bl_ui/properties_material_gpencil.py

@@ -40,6 +40,7 @@ class GPENCIL_MT_material_context_menu(Menu):
         ).only_active = False
 
         layout.operator("object.material_slot_remove_unused")
+        layout.operator("object.material_slot_remove_all")
 
 
 class GPENCIL_UL_matslots(UIList):

scripts/startup/bl_ui/space_view3d.py

@@ -3351,6 +3351,7 @@ class VIEW3D_MT_object_cleanup(Menu):
         layout.separator()
 
         layout.operator("object.material_slot_remove_unused", text="Remove Unused Material Slots")
+        layout.operator("object.material_slot_remove_all", text="Remove All Materials")
 
 
 class VIEW3D_MT_object_asset(Menu):