Showcase _scrollIntoView should ensure the showcase is visible, not its child

[mrsvan]

Describe the bug
When you wrap a tall widget in a "Showcase", with ShowCaseWidget(autoPlay: true), the showcase is not visible after the autoscroll.

To Reproduce
Steps to reproduce the behavior:

1. Wrap a widget in "Showcase", where the widget is taller than the current viewport;
2. Warap the Showcase in a ShowCaseWidget, with autoPlay: true;
3. When the showcase is triggered, the child widget will be scrolled into view (in the middle of the view)
4. The showcase itself is not visible, so the user does not understand what is being showcased;

Expected behavior
When the showcase is triggered, the explanation should scroll into view, as high up as possible in the viewport so that the child widget that is being showcased is also visible.

It is more important to see the entire showcase bubble, even if the child widget is only partially visible.

Desktop (please complete the following information):

• OS: all
• Browser all
• Version 3.0.0

Smartphone (please complete the following information):

• Device: all
• OS: all
• Browser all
• Version 3.0.0

Additional context
The current Showcase _scrollIntoView, uses alignment: 0.5, which forces the showcase to be scrolled out of view.