Select pre-existing URI's in URIComboBox

This makes it so when the popup is shown, the current URI is selected
and scrolled to instead of having nothing selected and the scroll area
is at the top of the list.

hab_gui/widgets/uri_combobox.py

@@ -40,4 +40,10 @@ def uri(self):
         return self.currentText()
 
     def set_uri(self, uri):
-        self.setEditText(uri)
+        # If the uri is already an item in the combo box, select it
+        index = self.findText(uri)
+        if index > -1:
+            self.setCurrentIndex(index)
+        else:
+            # Otherwise update the text of the combo box to match
+            self.setEditText(uri)