GPv3: Cleanup: Looping over frames in a layer

Use structured binding declaration to unwrap the item into `frame_number` and `frame`.
Makes the code more readable.

Pull Request: https://projects.blender.org/blender/blender/pulls/110525

source/blender/editors/grease_pencil/intern/grease_pencil_frames.cc

@@ -62,8 +62,7 @@ void select_all_frames(bke::greasepencil::Layer *layer, const short select_mode)
 
 bool layer_has_any_frame_selected(const bke::greasepencil::Layer *layer)
 {
-  for (auto item : layer->frames().items()) {
-    const GreasePencilFrame &frame = item.value;
+  for (const auto &[frame_number, frame] : layer->frames().items()) {
     if (frame.is_selected()) {
       return true;
     }