Implemented @sept-en's tip.
VisualizerEditor implements the listener class, with a dummy windowClosed() method.
Derivations of VisualizerEditor can optionally implement windowClosed().

This might be very Cyclops Stimulator specific addition, and there might be a better way to do this:
DataWindow is used only by VisulaizerEditors. So I added a callback to the editor whenever an (open) DataWindow is closed.
I don't know how useful this might be for plugins other than for Cyclops.

Motivation: Since the datawindow is also shared by CyclopsEditors, when a window is closed, only the editor which created the window would get it's windowSelector button toggled back to "off".

With this PR, the button toggles, and the editor's windowClosed callback is called, in which the message is passed to peers through an internally maintained structure -- so that they can toggle their buttons too.

These changes were made to primarily to accommodate for the cyclops-stimulator. Since buttonClicked manipulates the DataViewport via AccessClass, a sub-class of VisualizerEditor cannot functionally override the method.
Hence:

* Added protected methods which manipulate DataViewport (via AccessClass)
* moved the SelectorButtons into protected area, essential!
* All of the additions are essential to the cyclops-stimulator.
* There is NO change in the public interface.

Now, my buttonClicked method can access the SelectorButtons, manipulate (anything about!!) DataViewport.
Please remove the (new) comments from VisualizerEditor::buttonClicked (.cpp), they are not needed anymore.

Merge branch 'testing-visualizer' of https://github.com/arrow-/plugin-GUI into arrow--testing-visualizer

addToBuffer with >1 item

fix channel record state restoration

* Added docs for the class. Override only buttonEvent (as for other non-visual editors)
* Started fixing other editors, a lot of redundant code can be removed.
* See comments in VisualEditor.cpp

addresses #55

Back-port master fix to testing

Back merging master changes into testing