DR python scripts: one bugfix, one new

#3690: bugfix to select_all_models_of_type.py
Only 2 lines have changed, (the return value for Visitor's pre() ), but I can't get the diff viewer to show just that!

#3718: Script to find missing targets [New feature]
Forum discussion: http://forums.thedarkmod.com/topic/16059-pathtarget-testing-script-for-dr