The default config of i3 provides a key binding to reload, so changes
take effect immediately:
```
bindsym $mod+Shift+c reload
```
Unfortunately the current module uses the store path of the `configFile`
option. So when I change the config in NixOS, a store path will be
created, but the current i3 process will continue to use the old one,
hence a restart of i3 is required currently.
This change links the config to `/etc/i3/config` and alters the X
startup script accordingly so after each rebuild, the config can be
reloaded.
Now the default way to define NixOS window manager modules is to use
mkEnableOption to describe the module itself.
In this commit, all files on nixos/modules/services/x11/window-managers
are changed.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
