Clients::Studio: build fix

    * src/clients/studio/paditem.cpp: added missing include directive.
Clients::Studio: added possibility for adding elements

    * src/clients/studio/elementitem.cpp: throw an exception when model
      of element is empty (e.g. when element doesn't exist).
    * src/clients/studio/mainwindow.cpp: removed test elements,
      implemented "add element" button.
    * src/clients/studio/plugininspector.{cpp|h}: added method for getting
      currently selected item.