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/imode.cpp

@@ -5,6 +5,10 @@
 
 #include "modemanager.h"
 
+#include <utils/fancymainwindow.h>
+
+#include <aggregation/aggregate.h>
+
 namespace Core {
 
 /*!
@@ -111,6 +115,11 @@ void IMode::setEnabled(bool enabled)
     emit enabledStateChanged(m_isEnabled);
 }
 
+Utils::FancyMainWindow *IMode::mainWindow()
+{
+    return Aggregation::query<Utils::FancyMainWindow>(widget());
+}
+
 bool IMode::isEnabled() const
 {
     return m_isEnabled;