app: show the layer mode dropdown list properly.

Showing it was only displaying the top modes, with a lot of top space,
and you had to slowly scroll down the list. This is the same as #2642
(as Alexandre noted in a comment), so I just use the same "fix" though I
don't fully understand it. It feels more of a side effect of
gtk_combo_box_set_wrap_width() working around a bug of GtkComboBox. So
if anyone has a better fix and understand the issue, feel free to patch
(maybe GTK+ directly?). In the meantime, it works well enough. :-)

app/widgets/gimplayermodecombobox.c

@@ -143,6 +143,10 @@ gimp_layer_mode_combo_box_constructed (GObject *object)
 
   gimp_int_combo_box_set_active (GIMP_INT_COMBO_BOX (combo),
                                  combo->priv->layer_mode);
+  /* Sure why this is needed to have the dropdown display properly.
+   * Probably some weird side effect. See issue #2642.
+   */
+  gtk_combo_box_set_wrap_width (GTK_COMBO_BOX (combo), 1);
 }
 
 static void