* app/widgets/gimplayertreeview.c (gimp_layer_tree_view_layer_clicked):
  when ALT is pressed, select the layer's alpha. SHIFT and CONTROL work
  as usual to add, subtract and intersect.

* app/widgets/gimpchanneltreeview.c
* app/widgets/gimpvectorstreeview.c: add "clicked" handlers here too
  and do the same select-on-alt-click thing.