Update transition class to work more generally. Now it can be used for
showing and hiding popups.

* The containing object does not need to be a view stack, but can be a general Widget
* Can work with "toHide==null" or "toShow==null"
* Resets position and scale of moved object after animation
* Final position of "toShow" object is its original position and scale (where it is located when before animation is started)
* When there is a scaleUp transition, but nothing to show (toShow == null) it is made a scaleDown of the object to hide
added transitions to Options widget