Design mode/Qt Designer: Clean up the widget part of it.

- Remove shared subwindow reparenting from EditorWidget, make EditorWidget
  inherit FancyMainWindow. and use just once instance of it instead of
  per-editor ones.
- Embedded FormEditorStack into EditorWidget as a centralwidget.
- Changed FormWindowEditor's base class from IEditor to SharedTools::FormWindowHost
  (Widget) to be embedded into FormEditorStack (no need to be an IEditor),
  Remove Designer::Internal::FormWindowHost which had little functionality.
- Add Design Mode widget to FormEditorW which has FakeToolBar and EditorWidget
  (single instance) in a vertical layout.
- Removed ProxyAction class handling dock view toggle actions of several EditorWidgets
  (no longer necessary since there is just once instance). Moved "View menu" to bottom.
- Started to make FakeToolBar work as a single instance listening on changing
  xml editors
- Include-file/slot connection clean-up.