LibWeb+LibWebView+WebContent: Add support for <input type="color">

This commit introduces 3 things:
- Support for the color type in HTMLInputElement itself
- A mechanism for handling non event loop blocking dialogs in Page
- The associated plumbing up to ViewImplementation

Frontends may add support for the color picker with the
ViewImplementation.on_request_color_picker function

Userland/Libraries/LibWebView/ViewImplementation.cpp

@@ -207,6 +207,11 @@ void ViewImplementation::prompt_closed(Optional<String> response)
     client().async_prompt_closed(move(response));
 }
 
+void ViewImplementation::color_picker_closed(Optional<Color> picked_color)
+{
+    client().async_color_picker_closed(picked_color);
+}
+
 void ViewImplementation::toggle_media_play_state()
 {
     client().async_toggle_media_play_state();