[FEATURE] Add "visible" property to every Control

- The property "visible" has been added to sap.ui.core.Control
- In case visible is set to false, the renderer 
  RenderManager.InvisibleRenderer will be used, which only renders an 
  invisible placeholder element with the Control's ID and a placeholder
  prefix, instead of the Control's real renderer
- Invisible and Dummy ID prefixes have been moved to sap.ui.core
  library.js to be used as constants
- If a Control defines its own visible property, the real renderer
  will be used (to assure backwards compatibility)
- Invisible controls do not return anything on calling their getDomRef()
  method
- Testpage containing all Controls from our libraries that used to have
  a visible property

Change-Id: I1f975cc6363b15ab1bec6bdd63a716834cb7ef5c
Merge "[FEATURE] Add "visible" property to every Control"