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

@@ -1952,1 +1953,4 @@
     }
+    else if (rna_type == PROP_STRING) {
+      /* Not a delta in fact. */
+      RNA_property_string_get(&but->rnapoin, prop, delta.str);