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
Make _objWidth and _objHeight in Box public, so that they are accessible from outside
Allow layouts to have "0" entry, which makes the specific child get the size it would natively demand
Allow buttons to have "disabled" state
Merge branch 'layout_widget_native_size' of https://github.com/RudolfVonKrugstein/StablexUI into bugfix
Merge branch 'button_disabled' of https://github.com/RudolfVonKrugstein/StablexUI into bugfix
ignore mous events if button is disabled