toolbar: port GtkToolbar to GtkBox and add close button

also fix margin in RTL, and set the header bar as titlebar for the
window.

https://bugzilla.gnome.org/show_bug.cgi?id=706376
build: bump GTK requirement

For gtk_window_set_titlebar () and for
gtk_button_new_from_icon_name ().

https://bugzilla.gnome.org/show_bug.cgi?id=706376