Introduce kowidgetutils library and put KoGroupButton in it

The KoGroupButton's code is untouched (except for the EXPORT name) and comes from Jean-Nicolas Artaud - see https://git.reviewboard.kde.org/r/107530 so please do not report issues related to it here. In that patch KoGroupButton was put into kowidgets library what's suboptimal because of dependencies - no point to depend on kotext, etc. to use one unrelated widget.

This change is s small step to increase modularity.

More classes to come here, including improvements to KoGroupButton.

REVIEW:107842
DIGEST:Calligra increases modularity step by step