Creating New Toolbar Elements Buttons

[vgogo]

In the Toolbars Editor (Tools > Customize Interface), under Toolbar Elements are a bunch of buttons that can be dragged into the various toolbars.

I'd like to create an extension/plugin/script (whichever is the correct term) to extend the buttons to add new ones that I can incorporate into my toolbars.

For example, there's a "Frame By Frame" button which advances the video forward by one from, but I'd also like to create one that steps one frame backwards.

Is there any documentation or examples I could use to help me with this?