Add button in docked panels that cause them to be externally opened

[royerloic]

We have this button:

We could add another such button that opens a node externally (using the same machinery that is already at work). Now the difficulty here is that these buttons are handled in DockFX, so the right way to do it is to expose an API on the DockFX side that allows Halcyon to add such buttons. This way the actual handling is done on the halcyon side which knows which nodes are opened where and how...

Important: I would like to also have the possibility to open the toolbars externally by clicking on such a button. When a toolbar that is opened externally is closed, then it goes back 'home'.