Allow deciding to use the GtkHeaderBar at configure time (default: no)

Useful for platforms where we might prefer not to use this (e.g. OS X)
New UI file describing the menu model of the header's menu button
Use a GtkHeaderBar in the main window when USE_HEADER_BAR is defined

This should enable better integration with GNOME 3 from 3.10 on. It's
still possible not to use a header back and instead use a menu bar
plus a toolbar by passing --disable-header-bar at configure time.