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/Services/WebContent/WebContentServer.ipc

@@ -78,6 +78,7 @@ endpoint WebContentServer
     alert_closed() =|
     confirm_closed(bool accepted) =|
     prompt_closed(Optional<String> response) =|
+    color_picker_closed(Optional<Color> picked_color) =|
 
     toggle_media_play_state() =|
     toggle_media_mute_state() =|