Use sidebar buttons for hiding/showing dock areas

So far, if a mode includes navigation widget placeholders, these are
used for the sidebar toggle buttons, otherwise the buttons are disabled.

Now, if a mode does not include navigation widget placeholders, but the
mode has a FancyMainWindow attached, use the buttons to hide or show the
corresponding dock widget area (left or right).

Since QMainWindow does not really support "hiding a dock widget area",
the FancyMainWindow needs to track that state manually, by tracking the
dock widgets that were visible before "hiding the dock widget area".

Also, if a dock widget is dragged into a "hidden" area, or a widget is
made visible or "unfloated" into that area, show the other widgets in the
area again as well, "unhiding" the area.

Since the mode widgets that have a mainwindow somewhere usually wrap
that into a splitter for the output panes, and the Design mode is
actually a stack widget, IMode needs another method that returns the
appropriate FancyMainWindow if available.

The patch implements this for Widget Designer.

Change-Id: I03531f4d5130c846ff5d65831b1c9be210e1c561
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/coreplugin/navigationwidget.h

@@ -86,7 +86,7 @@ public:
     int storedWidth();
 
     // Called from the place holders
-    void placeHolderChanged(NavigationWidgetPlaceHolder *holder);
+    void placeHolderChanged();
 
     QHash<Utils::Id, Command *> commandMap() const;
     QAbstractItemModel *factoryModel() const;
@@ -96,7 +96,11 @@ protected:
 
 private:
     void closeSubWidget(Internal::NavigationSubWidget *subWidget);
-    void updateToggleText();
+    bool toggleActionVisible() const;
+    bool toggleActionEnabled() const;
+    bool toggleActionChecked() const;
+    void updateMode();
+    void updateToggleAction();
     Internal::NavigationSubWidget *insertSubItem(int position,
                                                  int factoryIndex,
                                                  bool updateActivationsMap = true);