Viewpoint icon is not displayed in odesign if the file it's a plugin file

[scosta-obeo]

Step to reproduce:

• Create a new File > New > Other > Ecore Modeling Project named “EcoreTest”
• Create a new viewpoint project (File > New > Other and select “Viewpoint Specification Project”)
• Open the .odesing file and select the viewpoint
• Click on the “...” button to set its icon
• Select the image from any plugins (but not in workspace)
• Expected: the icons is updated from in the VMS editor (in odesign)
• Expected: the viewpoint uses the selected icon (in aird)
• KO: the icon is update for aird but not odesign

Related to #363