[embed] Use GTK+ settings

[embed] Use GTK+ settings

When the style of GtkClutterEmbed changes we need to retrieve all
the settings and screen options used by GTK+.

This is needed because:

  - Clutter does not have any internal code to handle changes
    in the platform settings

  - GtkClutterEmbed "enslaves" Clutter to GTK+ already

By default, we reuse:

  - the screen resolution
  - the screen font options
  - the default font name
  - the double click distance and time

This way, a Clutter scene embedded into a GtkClutterEmbed will
look more integrated.