Allow to merge two classes together

Details: <p>Testing:</p><p>- Accessible through Catalogue Tools tab (A class must be open first.)</p><p>- &#39;Merge class&#39; button will display the list of classes allowing the user to choose the class which will be merged with the one open.</p><p>- If the selected class is a system object, operation should be canceled.&nbsp; (The class initially open can be a system object.)</p><p>- Both classes must allow to save Work objects associated to them. (Ex: You shouldn&#39;t be able to merge any class with the &#39;Root class&#39;...)</p><p>- Message waringin operation cannot be undone should be displayed.</p><p>- Class catalogue folders should be merged (folder should deleted and files moved to catalogue folder</p><p>- Opening a part belonging to the class that got merged should work and indicate it is now an object of the nature of the class it was merged with.</p>