home: wm: use explicit 'enable' options

This is so that I can explicitly enable them if I were to use Wayland
and Sway, given that they should be compatible with both environment.

This also means that putting them in `/home/x/` would a misnomer... Will
revisit this if and when I try out Wayland and sway...

home/wm/i3bar.nix

@@ -1,9 +1,9 @@
 { config, lib, pkgs, ... }:
 let
-  isEnabled = config.my.home.wm.windowManager == "i3";
+  cfg = config.my.home.wm.i3bar;
 in
 {
-  config = lib.mkIf isEnabled {
+  config = lib.mkIf cfg.enable {
     home.packages = with pkgs; [
       alsaUtils # Used by `sound` block
       lm_sensors # Used by `temperature` block