package/matchbox: enable support for the Xsession manager

Since this is a non-obvious dependency, just select the appropriate
library.

Also, add a comment to act as a separator between matchbox' options
and the other matchbox packages.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>