Allow users to assign unique icons to custom commands in the toolbar.

Currently, all custom commands/actions in a tool bar use the same icon so the user must remember the order they are in and/or mouseover the icon to see the tooltip in order to confirm they are clicking on the right command.

It would be nice if we could choose from, say, a list of 25 or 50 icon images and assign them to the actions as a way of helping to quickly identify which is which (kind of like how Microsoft's quick access toolbar does in Word/Excel, etc).