A MovableContainer can have its content moved on screen with Swipe/Hold/Pan.
Can optionally apply alpha transparency to its content.
With Swipe: the widget will be constrained to screen borders. With Hold and pan, the widget can overflow the borders.
Hold with no move will reset the widget to its original position. If the widget has not been moved or is already at its original position, Hold will toggle between full opacity and 0.7 transparency.
This container's content is expected to not change its width and height.