added wm_class property

[ivanvig]

Right now i3lock has only a wm_name property but lacks of wm_class which is useful to, for example, set compton rules for i3lock's window.

[Airblader]

Thanks for the PR! I'm definitely in favor of adding this, but I think we could do entirely without the new macros here as they just add a level of redirection when reading the code that we don't need. Could you remove them and just use string literals instead? Be carefull for the length of "i3lock\0i3lock\0" to use 2 * (strlen("i3lock") + 1).

Also, please squash the commits into a single commit (and force-push the branch) and drop a comment here so that we get notified. Thank you!

[ivanvig]

I think is done, went back to master and only added the code block corresponding to the property

[Airblader]

Thank you!