Fix updates of undo widget in styling panel

Before, on current layer change it would switch to undo stack
of freshly activated layer, ignoring the layer set in the combo box
(and the undo stack it switched to was the one for editing)

src/app/qgisapp.cpp

@@ -475,6 +475,19 @@ void QgisApp::activeLayerChanged( QgsMapLayer* layer )
 {
   if ( mMapCanvas )
     mMapCanvas->setCurrentLayer( layer );
+
+  if ( mUndoWidget )
+  {
+    if ( layer )
+    {
+      mUndoWidget->setUndoStack( layer->undoStack() );
+    }
+    else
+    {
+      mUndoWidget->destroyStack();
+    }
+    updateUndoActions();
+  }
 }
 
 /**

src/app/qgsundowidget.cpp

@@ -14,9 +14,8 @@
  ***************************************************************************/
 #include "qgsundowidget.h"
 
-#include "qgisapp.h"
 #include "qgsapplication.h"
-#include "qgslayertreeview.h"
+#include "qgslogger.h"
 #include "qgsmaplayer.h"
 #include "qgsmapcanvas.h"
 
@@ -28,8 +27,6 @@ QgsUndoWidget::QgsUndoWidget( QWidget * parent, QgsMapCanvas * mapCanvas )
 
   connect( undoButton, SIGNAL( clicked() ), this, SLOT( undo() ) );
   connect( redoButton, SIGNAL( clicked() ), this, SLOT( redo() ) );
-  connect( QgisApp::instance()->layerTreeView(), SIGNAL( currentLayerChanged( QgsMapLayer* ) ),
-           this, SLOT( layerChanged( QgsMapLayer* ) ) );
 
   undoButton->setDisabled( true );
   redoButton->setDisabled( true );
@@ -42,20 +39,6 @@ QgsUndoWidget::QgsUndoWidget( QWidget * parent, QgsMapCanvas * mapCanvas )
 }
 
 
-void QgsUndoWidget::layerChanged( QgsMapLayer * layer )
-{
-  if ( layer )
-  {
-    setUndoStack( layer->undoStack() );
-  }
-  else
-  {
-    destroyStack();
-  }
-  emit undoStackChanged();
-}
-
-
 void QgsUndoWidget::destroyStack()
 {
   if ( mUndoStack )

src/app/qgsundowidget.h

@@ -64,10 +64,6 @@ class APP_EXPORT QgsUndoWidget : public QgsPanelWidget
     QWidget* dockContents() { return dockWidgetContents; }
 
   public slots:
-    /**
-     * Changes undo stack which is displayed by undo view
-     */
-    void layerChanged( QgsMapLayer * layer );
 
     /**
      * Slot to handle undo changed signal