Fix state of side bar buttons when switching different design widgets

When switching between different editors in Design mode (e.g. ui and
qml), the side bar buttons must update. So far they reacted on mode
switches, but not on changes of the main window while staying in the
same mode.

Change-Id: I85635f12bb8613e90ec81056d8763431c04fea2e
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/coreplugin/modemanager.cpp

@@ -322,6 +322,7 @@ void ModeManager::currentTabChanged(int index)
         oldMode = d->m_modes.at(d->m_oldCurrent);
     d->m_oldCurrent = index;
     emit currentModeChanged(mode->id(), oldMode ? oldMode->id() : Id());
+    emit currentMainWindowChanged();
 }
 
 /*!