Move setNewFolderShortcutAction() call inside ctor
In 2 out of 3 cases, the constructor of DolphinNewFileMenu has all information it needs to setup its newFolderShortcutAction itself. So let the ctor handle this instead of letting the user of the class deal with it.
In the 1 case out of 3 were this doesn't work, the caller of the class still has to still call setNewFolderShortcutAction() themself.