Added support for easier layout selection

[JezerM]

Closes #6

Changes

• Adds layouts section in /etc/lightdm/web-greeter.yml
• Layouts accesible through greeter_config.layouts
  • Returns an array of LightDM.Layout objects, only if they are also inside lightdm.layouts

Setting the config like this:

layouts:
  - us
  - latam
  - not valid layout

... will return:

> greeter_config.layouts
[
  { name: "us",    description: "", ... },
  { name: "latam", description: "", ... }
]