UI: enable string properties for alt-click buttons for multiple objects

This came up in #106591 which reported that changing a Light Group would
not work when alt-clicking the property field (which is the usual method
to edit a property for multiple objects at once).

This is because string properties were not supported in
`ui_selectcontext_apply` which is now done.

Similar to 1318660b04 [which added support for pointer properties].

Adding general support for string properties means this method can now
be used for many more things:
- changing all sorts of ID names (objects, meshes, ...)
- many settings in modifiers (e.g. vertexgroups)
- geometry nodes modifier properties (e.g. attribute names)
- ...

Fixes #106591

Pull Request: https://projects.blender.org/blender/blender/pulls/106599

source/blender/editors/interface/interface_handlers.cc

@@ -1918,6 +1918,7 @@ static void ui_selectcontext_apply(bContext *C,
       bool b;
       int i;
       float f;
+      char *str;
       PointerRNA p;
     } delta, min, max;
 
@@ -1950,6 +1951,10 @@ static void ui_selectcontext_apply(bContext *C,
       /* Not a delta in fact. */
       delta.p = RNA_property_pointer_get(&but->rnapoin, prop);
     }
+    else if (rna_type == PROP_STRING) {
+      /* Not a delta in fact. */
+      delta.str = RNA_property_string_get_alloc(&but->rnapoin, prop, nullptr, 0, nullptr);
+    }
 
 #  ifdef USE_ALLSELECT_LAYER_HACK
     /* make up for not having 'handle_layer_buttons' */
@@ -2023,9 +2028,16 @@ static void ui_selectcontext_apply(bContext *C,
         const PointerRNA other_value = delta.p;
         RNA_property_pointer_set(&lptr, lprop, other_value, nullptr);
       }
+      else if (rna_type == PROP_STRING) {
+        const char *other_value = delta.str;
+        RNA_property_string_set(&lptr, lprop, other_value);
+      }
 
       RNA_property_update(C, &lptr, prop);
     }
+    if (rna_type == PROP_STRING) {
+      MEM_freeN(delta.str);
+    }
   }
 }