gui refactoring: rename MicroscopeGUIModel to instrmodel.MicroscopyGUIData

trying to avoid confusion it's a microscope (or a representation of it). It's the data model for
the View and Controllers used in Miscroscopy GUI.
gui refactoring: separate global GUI model from per tab model

Now there is only one instance of the object containing info on the microscope (MainGUIData)
Each tab has its own instance on the views and streams.

It makes things closer to our mental model. It also avoids to scan the microscope several time at initialisation.
Finally, it simplifies back the on/off buttons controller to its original code.