Allow changing the GTK class name using `--class`

[bergercookie]

Hi there,

Many GUI programs allow you to change the WM_CLASS of this program using a --class argument, e.g., see gnome-terminal. This would be useful e.g., in i3 for assigning specific programs to specific workspaces and having a fine-grained control over this.

Thanks for this awesome project Nikos

[bergercookie]

Hi @vhakulinen, friendly ping :)