Graph widget: add "Export to GML" button.
Merge branch 'master' of github.com:acardona/CATMAID
Remove unused import.
Speed up two functions from common.py.

In tests, manually building a dictionary with a loop or doing it with dict
from a list of tuples consumes twice the time than using dictionary comprehension.

Additionally, there's no need to request all fields of Class or Relation,
neither build objects for each to represent them.
Speed up node_tuples_list by removing checks for the existenceof classes
and relations--if it fails, it fails. This function is performance-critical.

Thanks to Tom Kazimiers for making me realize that these tests where
still present in node_tuples_list. These were left over from PHP times.