osx: Set Menlo as default font

This moves the find_font function to the front ends since it depends on
pango and cairo

src/fe-text/fe-text.c

@@ -913,3 +913,8 @@ fe_open_chan_list (server *serv, char *filter, int do_refresh)
 {
 	serv->p_list_channels (serv, filter, 1);
 }
+const char *
+fe_get_default_font (void)
+{
+	return NULL;
+}