Add support for defining custom C++ class templates via UI

Currenty, C++ class templates are hardcoded in <app-bundle>/Contents/Resources/templates/wizards/classes/cpp and I am able to replace those with my custom templates and this works correctly (I can even modify the wizard.json to add additional check boxes, like e.g. for auto-creating copy and move constructors and assignment operators).

However, whenever I update QtCreator, I have to patch those files with my version, since Qt Creator does not appear to load those templates from user config. Furthermore, it would be great if such templates could be customized via UI (with some built-in editor, like one for license template).