Fix T101334: Paint Texture Slots appear greyed out

To the user, this looks like a disfunctional thing (usually greying out
is used for props having no effect).

The greying out is caused by
{rB8b7cd1ed2a17e40661101eea4adae99e8e3d02e9}.
Above commit disabled the direct renaming of images in the
`TEXTURE_UL_texpaintslots` UIList (and instead displays the texture slot
directly as a prop -- which has its `PROP_EDITABLE` flag cleared)
(from the commit message):
> A limitation of this patch is that is isn't possible anymore to rename
images directly from
> the selection panel. This is currently allowed in master. But as
CustomDataLayers
> aren't ID fields and not owned by the material supporting this wouldn't
be easy.

To work around the UI confusion (but still keep the non-editable nature
of the property), now just display this as a label.

Maniphest Tasks: T101334

Differential Revision: https://developer.blender.org/D16138

release/scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -278,7 +278,7 @@ class TEXTURE_UL_texpaintslots(UIList):
         # mat = data
 
         if self.layout_type in {'DEFAULT', 'COMPACT'}:
-            layout.prop(item, "name", text="", emboss=False, icon_value=item.icon_value)
+            layout.label(text=item.name, icon_value=item.icon_value)
         elif self.layout_type == 'GRID':
             layout.alignment = 'CENTER'
             layout.label(text="")