Prompt before replacing an existing map preset

It's too easy to accidently (and unrecoverably) erase an
entire preset using the replace preset menu

(cherry-picked from b1158ff)

src/app/qgsvisibilitypresets.cpp

@@ -30,7 +30,7 @@
 #include "qgsnewnamedialog.h"
 
 #include <QInputDialog>
-
+#include <QMessageBox>
 
 QgsVisibilityPresets* QgsVisibilityPresets::sInstance;
 
@@ -181,6 +181,12 @@ void QgsVisibilityPresets::replaceTriggered()
   if ( !actionPreset )
     return;
 
+  int res = QMessageBox::question( mMenu, tr( "Replace preset" ),
+                                   tr( "Are you sure you want to replace the existing preset %1?" ).arg( actionPreset->text() ),
+                                   QMessageBox::Yes | QMessageBox::No, QMessageBox::No );
+  if ( res != QMessageBox::Yes )
+    return;
+
   //adding preset with same name is effectively a replace
   addPreset( actionPreset->text() );
 }