HTML Widgets; widget.UI.Block; Unblock now delayed to prevent quick black flashes

When an HTML Widget is loading content (e.g., via widget.GetString), it typically blocks the User Interface by calling widget.UI.Block. Once the content has been retrieved, widget.UI.Unblock is called.

Since blocking the User Interface involves showing a black overlay with a message and wait icon, on fast connections or for small requests, it can appear that the screen flashes black which can be confusing.

Now, when widget.UI.Unblock is called, it will be delayed to ensure that the message has been displayed for at least 0.75 seconds.