Port Pitivi to Python 3
tests: Make the 'mock missing' reported error more readable
medialibrary: Use proper API to get uris from dragging context
medialibrary: Do not sanitize URIs it is not needed aymore

This broke after the pass to the py2to3 script and we actually do not
need it as we are now using DragContext.get_uris() to get the URI
and this provide us with a clean list of ready to be used URIs
loggable: Windup the stack leads to wrong frames
mainwindow: Replace the menubar and main toolbar by HeaderBar and MenuButton

Fixes bug #708375
Depends on GTK+ 3.10

This commit removes any dependency on GTK actions for the main window. It also
changes translatable strings and moves the "screenshot" feature to mainwindow.
dogtail: Fix some
mainwindow: Allow pressing F10 for opening the app menu
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