Make Pitivi a GtkApplication

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=693291
Use Gio.SimpleActions to handle global app keyboard events
dogtail: Use keyboard combos to workaround unsniffable popup menu
dogtail: Fix test_timeline.py:TimelineTest.test_multiple_drag
dogtail: Fix test_timeline.py:TimelineTest.test_split
dogtail: Update timeline tests to use the new getTimelineX/Y methods
dogtail: Fix first half of test_timeline.py:TimelineTest.test_ripple_roll
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