Use signal/slots instead of deep function calls into decoration

For all the decoration updates called from Client into the decoration we
also have a signal being emitted. So turning the pure virtual public
functions into slots means we can just connect our existing signals and
get rid off the deep function calls.

The keepAbove/Below signals are changed to take a boolean argument as
needed by KDecoration and a few emitted signals are moved to a better
fitting location.

REVIEW: 110335