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 using Ctrl-Q to close the app
mainwindow: Allow pressing F10 for opening the app menu
Make Pitivi a GtkApplication

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=693291