Before this PR Lem SDL2 has various WM_CLASS like sbcl, lem-sdl2, etc... It caused X11's WM_CLASS is not set explicitly. Because of it we cannot configure some software detect Lem process with WM_CLASS e.g. xkeysnail.

This PR sets WM_CLASS as Lem SDL2 so now users can be find Lem SDL2 process via windos class name.

I checked WM_CLASS is set after this commit like this:

;; clicked lem window after launching `lem-sdl2` or `lem -f sdl2` or something
$ xprop | grep WM_CLASS
WM_CLASS(STRING) = "Lem SDL2", "Lem SDL2"