Fully support <input type=color> (#36992)

This change adds a shadow-tree widget for `<input type=color>` elements.
It also involves some changes to the way layout interacts with the DOM,
because currently all `input` and `textarea` elements are rendered as
plain text and their descendants are ignored. This obviously doesn't
work for `<input type={color, date, range, etc}>`.


![image](https://github.com/user-attachments/assets/4f16c3b0-1f79-4095-b19d-1153f5853dd5)

<details><summary>HTML used for the screenshot above</summary>

```html
<input type=color>
```

</details>



Testing: I doubt that this affects WPT tests, because the appearance and
behaviour of the widget is almost entirely unspecified.

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

ports/servoshell/desktop/app_state.rs

@@ -609,6 +609,15 @@ impl WebViewDelegate for RunningAppState {
                 let offset = self.inner().window.toolbar_height();
                 self.add_dialog(webview, Dialog::new_select_element_dialog(prompt, offset));
             },
+            FormControl::ColorPicker(color_picker) => {
+                // FIXME: Reading the toolbar height is needed here to properly position the select dialog.
+                // But if the toolbar height changes while the dialog is open then the position won't be updated
+                let offset = self.inner().window.toolbar_height();
+                self.add_dialog(
+                    webview,
+                    Dialog::new_color_picker_dialog(color_picker, offset),
+                );
+            },
         }
     }
 }