Use a headerbar instead of a toolbar

https://bugzilla.gnome.org/show_bug.cgi?id=720034
Depend on GTK+ 3.11.2

For GtkHeaderbar, including the fallback app menu.

https://bugzilla.gnome.org/show_bug.cgi?id=720034
Depend on Vala 0.23

For GTK+ 3.11.2 API

https://bugzilla.gnome.org/show_bug.cgi?id=720034
Conditionally display the New button in headerbar

If the app menu is displayed in the window, then this button is
redundant.

https://bugzilla.gnome.org/show_bug.cgi?id=720034