Greeter: theme packages

[kavau]

Greeter themes are currently limited to overriding CSS properties. From a user's
perspective, however, a theme can contain anything that influences the greeter's
appearance. Consider offering theme packages:

A theme package is a directory containing some or all of the following:

• theme.css (standard css overrides)
• theme.ini (overriding a subset of config keys such as font-size and
  background-image)
• background image(s), perhaps in a wallpaper/ subfolder

If the theme= key in the greeter config is set to a directory, atrium-greeter
will look for these elements and apply them as overrides to the config and
built-in css.