Added color chooser, gradient chooser and pattern chooser to the style preview docker. When changing the style with the style buttons, the respective chooser is shown on the right side.

Added a new gradient item chooser to ui/widgets wich is now used in the gradient tool and the style docker.

Moved the pattern item and pattern item chooser from plugins/tools to ui/widgets

There are still a few rough edges but one can already see in what direction it goes.