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/LibWeb/HTML/HTMLInputElement.h

@@ -85,6 +85,8 @@ public:
 
     bool is_mutable() const { return m_is_mutable; }
 
+    void did_pick_color(Optional<Color> picked_color);
+
     JS::GCPtr<FileAPI::FileList> files();
     void set_files(JS::GCPtr<FileAPI::FileList>);