DUI: Bug Tool-tip remains visible(switch to another program)

# Summary
It's small PR.
Bug was fixed and I have added comments, so that it's clear what I have done. So, I will just copy these comments here.

When we create LibraryToolTipPopup, we also add `Deactivated` event to the main window.
```c#
// If window is no longer on top, then we can hide tooltip.
// It's used, when we switch to another program.
 Application.Current.MainWindow.Deactivated += CloseLibraryToolTipPopupImmediately;
```

I used another method, that I called `CloseLibraryToolTipPopupImmediately`.
```c#
// We can not use CloseLibraryToolTipPopup, 
// because it waits for 1 sec to close itself,
// and we need to close tooltip immediately, 
// at the same moment when we switch to another program. 
private void CloseLibraryToolTipPopupImmediately(object sender, EventArgs e)
{
      this.DataContext = null;
}
```

# Reviewers
@Benglin, please take a look.
I would suggest the following modifications (and don't use ```/*```, use the double slash instead):

```cs
// When Dynamo window goes behind another app, the tool-tip should be hidden right 
// away. We cannot use CloseLibraryToolTipPopup because it only hides the tool-tip 
// window after a pause.
```