Allow to display login form on selected monitors only

[nwg-piotr]

• Added the "form_on_monitors" config key, equal [] by default. Use a list, e.g. [0, 1] to specify which monitors to display the login form on. The rest will only display the wallpaper. Closes #12; the nwg-hello-default.json file looks like this from now on:

{
  "session_dirs": [
    "/usr/share/wayland-sessions",
    "/usr/share/xsessions"
  ],
  "custom_sessions": [
    {
      "name": "Shell",
      "exec": "/usr/bin/bash"
    }
  ],
  "monitor_nums": [],
  "form_on_monitors": [],
  "delay_secs": 1,
  "cmd-sleep": "systemctl suspend",
  "cmd-reboot": "systemctl reboot",
  "cmd-poweroff": "systemctl poweroff",
  "gtk-theme": "Adwaita",
  "gtk-icon-theme": "",
  "gtk-cursor-theme": "",
  "prefer-dark-theme": true,
  "template-name": "",
  "lang": "",
  "env-vars": []
}

• the install.sh script changed, in order not to use setup.py any longer. Make sure you have 'python-build', 'python-installer', 'python-wheel' and 'python-setuptools' installed.