GroupItem may not be selected or focused

Only its children can be.  This fixes the problem, that a group item
could even be selected by clicking somewhere between the item's children
and the upper left corner of the scene (because the group items bounding
rect extends all the way up to (0,0))

sloth/items/items.py

@@ -416,6 +416,8 @@ class GroupItem(BaseItem):
         self._children = []
         BaseItem.__init__(self, model_item, prefix, parent)
         self.setFlag(QGraphicsItem.ItemIsMovable, False)
+        self.setFlag(QGraphicsItem.ItemIsSelectable, False)
+        self.setFlag(QGraphicsItem.ItemIsFocusable, False)
 
         self.createChildren()