dogtail: Fix and update test_timeline.py:TimelineTest.test_ripple_roll_image_video_mix to detect properly the video sample
dogtail: Look for timecode_widget in TimelineTest.setUp
dogtail: Merge test_base and helper_functions into a single PitiviTestCase class
settings: Fix overriding settings using environment variables
dogtail: Tell the app to use a temporary directory for xdg user dirs