The event is signalled in toolbar_JSdefines() in inc/toolbar.php before and after the standard toolbar is being filled. You can use this event for adding additional buttons to the toolbar.
$data contains an array of already defined toolbar buttons. Each element of the array may contain keys such as:
type — Type of action, see below.All types use:
title — Hover texticon — Image for the buttonkey — The shortcut key is the platform equivalent of ALT, plus this letter
format types use:
open — Text to place before cursor/selectionclose — Text to place after cursor/selectionsample — ??
insert types use:
insert — Text to insert
mediapopup types use:
url — Popup URLname — Popup window name?options — Added to URL?
picker types use:
list — function with optionsicobase — ??
signature types use:
The array is coverted to JSON, and passed to /lib/scripts/edit.js
The following plugins are known to handle this event and their source code may be a good start for understanding and implementing a handler yourself.