Add an option to set the window class name

[tzeikob]

Wouldn't it be useful to have an option to set the WM_CLASS, WM_NAME properties the same way other terminals do, for instance in alacritty we can do this:

alacritty --class scratch,scratch

--class <GENERAL> | <GENERAL>,<INSTANCE>

Defines the window class hint on Linux.
When only the general class is passed, instance will be set to the same value.

On Wayland the general class sets the app_id, while the instance class is ignored.

Default: Alacritty,Alacritty