Debugger: Drop empty lines at begin of copied backtrace

Fixes: QTCREATORBUG-26749
Change-Id: I7ecd85e45918f2fc70ad76fc3b977045a893f1d5
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/stackhandler.cpp

@@ -384,7 +384,7 @@ static QString selectedText(QWidget *widget, bool useAll)
         }
     }, model, QModelIndex());
 
-    return str;
+    return str.trimmed();
 }
 
 // Write stack frames as task file for displaying it in the build issues pane.