tabsmanager: Fix utility windows' shifting behaviour

When using event.x/y, the saved values would be the internal window's position.
To save the general window position, window.get_position() was used.

Fixes bug #692980
mainwindow: Fix main window's panes shifting position

Complements the fix for utility windows in bug #692980
python3: port pre-commit hook to python 3

add pep8 arch linux package name.
python3: change shebangs
python3: update build

* don't set minor version specificly
* use AM_CHECK_PYTHON_HEADERS for headers
* use hack from gst-python and pygobject for .so files
* remove AX_PYTHON_DEVE
python3: apply 2to3
python3: logger string/bytes convertion
python3: port more strings

* convert cairo masks to bytestring
* split string unstatically
* fix "Unicode-objects must be encoded before hashing"
* don't encode string for gtk
python3: port coptimizations renderer module
python3: apply 2to3 to tests

* fix test imports from 2to3
python3: fix tests
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