fixed discard or keep combobox on Mac

Use the new controls ComboBox

resources/qml/Dialogs/DiscardOrKeepProfileChangesDialog.qml

@@ -3,6 +3,7 @@
 
 import QtQuick 2.1
 import QtQuick.Controls 1.1
+import QtQuick.Controls 2.15 as NewControls
 import QtQuick.Dialogs 1.2
 import QtQuick.Window 2.1
 
@@ -145,10 +146,11 @@ UM.Dialog
         anchors.margins: UM.Theme.getSize("default_margin").width
         height: childrenRect.height
 
-        ComboBox
+        NewControls.ComboBox
         {
             id: discardOrKeepProfileChangesDropDownButton
             width: 300
+            textRole: "text"
 
             model: ListModel
             {
@@ -217,4 +219,4 @@ UM.Dialog
             }
         }
     }
-}
+}