Prompt before replacing an existing map theme

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

Author: nyalldawson

src/app/qgsmapthemes.cpp

@@ -29,7 +29,7 @@
 #include "qgsnewnamedialog.h"
 
 #include <QInputDialog>
-
+#include <QMessageBox>
 
 QgsMapThemes* QgsMapThemes::sInstance;
 
@@ -136,6 +136,12 @@ void QgsMapThemes::replaceTriggered()
   if ( !actionPreset )
     return;
 
+  int res = QMessageBox::question( mMenu, tr( "Replace theme" ),
+                                   tr( "Are you sure you want to replace the existing theme “%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() );
 }