Add i3wm as a possible window manager to Mate

[kafran]

i3wm integrates seemingly well to Mate, at least until Mate 1.22 (Mint 19.3) as Mate is become more like Gnome and less like Mate. I changed mintdesktop to offer the possibility to change the WM to i3 on Mate https://github.com/kafran/mintdesktop

As I think the i3 own panel (i3bar) integrates with i3 better than Mate's one, the window-manager-launcher will kill the mate-panel and change "org.mate.session.required-components-list" to ["windowmanager"] and "org.mate.background.show-desktop-icons" to False. If back to other WM it restores everything to default:

if current_desktop == "MATE" and wm != "i3":
    settings = Gio.Settings("org.mate.session")
    settings.reset("required-components-list")
    settings = Gio.Settings("org.mate.background")
    settings.reset("show-desktop-icons")
    subprocess.Popen(["mate-panel", "--replace"])

I don't know how to make this behave the most general/transparent as possible to maybe have this ever integrated into Mint. I use this setup for years as Mint Mate is the best Desktop to work on i3: