SubsurfaceAbout class cleanup.

Main change is that now SubsurfaceAbout doesn't have instance object, but
it constructs and destructs dynamically.

 * Remove the static SubsurfaceAbout::instance() class member
 * construct/destruct about dialog on demand
 * Other small aboutbox cleanups.

Signed-off-by: Boris Barbulovski <bbarbulovski@gmail.com>
ACK-ed-by: Tomaz Canabrava <tcanabrava@kde.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>