Add welcome page as dock widget.

The welcome page is based on the file welcome.docbook
which is converted into a html file on building.

The implementation has support for loading a translated
welcome file installed in the directory where other umbrello
related docbook files lives.

BUG:372252


FIXED-IN:2.20.80 (KDE Applications 16.11.80)

Signed-off-by: Ralf Habacker <ralf.habacker@freenet.de>