#44 Change CellDesigner Validation GUI

[Hugo Giesteira]

The Export CellDesigner Layout GUI needs to be changed.
First the operation should not be enable when there are no CellDesigner items in the clipboard. It is enable if a generic layout is present.
Also the validation is not working well, for instance, despite non of the projects have CellDesigner layouts the OK button is enable and when selected a warning is shown saying that there are no CellDesigner layouts in the selected project.