Update org.kde.kdenlive.desktop to inherit correct window title and icon

Tested with Kdenlive 20.04.0 on latest Arch. The application title and icon were correct in the GNOME Applications view, but upon opening Kdenlive, the window title was "kdenlive" and the window icon did not match my system Kdenlive icon, which is provided by Papirus (but this applies to other themes that provide a Kdenlive icon). Adding a startup WM class ensures the window title is capitalised the the window icon matches the user's application icon.