Use our usual background color in the Greeter

On the Wheezy builds, the Greeter displays the default Debian Wheezy’s background image. We should replace it with the usual blue color that is used on the desktop.

Feature Branch: bugfix/7126-gdm-background

Related issues

Blocks #7187 (closed)

Original created by @intrigeri on 7126 (Redmine)

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