Introduce QXcbXSettings

This allows you to easily retrieve properties in the XSETTINGS
specification. It is also possible to add listeners to get notified when
a specific property changes.

XSETTINGS is lazy initialized, so it will not be instansiated before
someone uses it. For now the intended use is a fallback for finding
cursor theme

Change-Id: Id47f0613f5876424cd47d721b40da17d3f63429e
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>