tooltip: fix line wrapping

We enable line wrap on the internal GtkLabel, but nowadays this does
nothing unless we also set a max number of characters.

https://bugzilla.gnome.org/show_bug.cgi?id=737929

gtk/gtktooltip.c

@@ -205,6 +205,7 @@ gtk_tooltip_init (GtkTooltip *tooltip)
 
   label = gtk_label_new ("");
   gtk_label_set_line_wrap (GTK_LABEL (label), TRUE);
+  gtk_label_set_max_width_chars (GTK_LABEL (label), 70);
   gtk_box_pack_start (GTK_BOX (box), label, FALSE, FALSE, 0);
 
   tooltip->window = window;