Implemented some toolbar event triggering

`toolbaritemclick`: Triggered when a button in the toolbar is clicked
`toolbarshown`:     Triggered when the toolbar is shown
`toolbarhidden`:    Triggered when the toolbar is hidden

Some times is very handy to know when the toolbar is being shown/hidden. For instance, I use the toolbar to provide formatting options to an item, and then save the item's style to the server when the toolbar is closed.

The `toolbaritemclick` is handy when the toolbar content is generated (like providing raw HTML), and there is no DOM node to register for events.