enable/disable menu items and toolbar items as appropriate
It is standard design practice to enable/disable menu and toolbar items as
appropriate. Matthew mentioned this in bug 2246. I haven't suggested this as
of yet because often it involves quite a bit of additional logic to the code.
But Matthew's mention reminded me of it and it does deserve its own bug.
Enabling/disabling menu/toolbar items is very useful to users in helping them
understand what they can do in certain contexts and with certain objects, and
it also helps decrease menu/toolbar complexity.