dnd_source: Add suppport for surface offset

The `drag_icon` callback is passed the offset of the cursor within the
widget at the start of the drag, and can return an offset the drag
surface should be placed relative to the cursor.

src/widget/text_input/input.rs

@@ -1350,12 +1350,13 @@ where
                                     clipboard,
                                     false,
                                     id.map(|id| iced_core::clipboard::DndSource::Widget(id)),
-                                    Some((
+                                    Some(iced_core::clipboard::IconSurface::new(
                                         Element::from(
                                             TextInput::<'static, ()>::new("", input_text.clone())
                                                 .dnd_icon(true),
                                         ),
                                         iced_core::widget::tree::State::new(state_clone),
+                                        Vector::ZERO,
                                     )),
                                     Box::new(TextInputString(input_text)),
                                     DndAction::Move,