Improve check for selected lithophane image

Checks for a Proxy attribute before accessing it and validates that the
proxy is an actual lithophane image by checking for the special attribute.

fixes #13

lithophane_image.py

@@ -174,7 +174,7 @@ def __init__(self, obj, imagePath):
         obj.Proxy = self
 
         self.lastPath = imagePath
-        self.pointCloudName = None
+        self.isLithophaneImage = True
 
     def execute(self, fp):
         '''Recompute the image when something changed'''

lithophane_utils.py

@@ -11,15 +11,15 @@ def findSelectedImage():
   selection = FreeCADGui.Selection.getSelection()
 
   if len(selection) != 1:
-    return None
+    return (None, None)
 
   selectedObject = selection[0]
 
-  if selectedObject.Proxy is None:
-    return None
+  if not hasattr(selectedObject, 'Proxy') or selectedObject.Proxy is None:
+    return (None, None)
 
-  #if not isinstance(selectedObject.Proxy, LithophaneImage):
-  #  return None
+  if not hasattr(selectedObject.Proxy, 'isLithophaneImage') or not selectedObject.Proxy.isLithophaneImage:
+    return (None, None)
 
   return (selection[0].Proxy, selection[0].Label)