app menu: standardize help/about/quit

As of 3.12, we are using this style fairly consistently.

https://bugzilla.gnome.org/show_bug.cgi?id=731312
Move the board size preference to the app menu

We're going to treat this as an application-level preference.

https://bugzilla.gnome.org/show_bug.cgi?id=731312
Replace the menu bar with a header bar

Depends on GTK+ 3.13.2 for RTL icon theme support.

https://bugzilla.gnome.org/show_bug.cgi?id=731312
Replace GtkVBox with GtkBox

GtkVBox has been deprecated

https://bugzilla.gnome.org/show_bug.cgi?id=731312