pass current document to views, so they work on this document, not with global pointers and MainWindow

...instead via MainWindow

"WidgetBox" was due to widget selection in QtDesigner, from which this class was copied once

This is more UI standard than the combo in bottom right corner.

As a consequence, hide the always empty status bar (for all views) and with this win more space for relevant content.

* reduce indirections
* reduce redundancy
* rm sample view actions class (had barely any functionality)
* rm sample view docks class (integrated into sampleview)