Add contribution point for tool palette toolbar items

[martin-fleck-at]

The current tool palette loads it's items through the RequestContextActions with contextId: ToolPalette.ID. This is nice and extensible. However, the tool bar items on top of the palette are hard-coded in the rendering of the tool palette UI extension. It would be great to allow adopters (and our own modules for that matter) to provide an additional toolbar items dynamically.