Refactoring of SiteIdInterface test

* Smaller but more tests with improved structure.
* Removed invalid test (added TODO/FIXME) which made false assumptions about
  wb.ui.PropertyEditTool.EditableValue.Interface interface.
* Some documentation clarifications.

Change-Id: I1a625c1fed05bd85031c60f1ec0008b8a5951c89
Merge "Refactoring of SiteIdInterface test"