IMPORTANT: the "sap.ui.core.Popup" is not a visible control!!
It is a helper that works in the background and can be used to convert any control
or any HTML element into a popup that sits on top of the rest of the screen and can be modal or can auto-close when losing the focus.
While this test page uses SAPUI5 controls to pop up, there is a similar page in sap.ui.core which pops up plain HTML elements.