Add ctkWorkflowButtonBoxWidget::[back|next|goTo]ButtonFormat

Give the option to dynamically configure the text, icon and tooltip
of the back, next and finish buttons.

Expose ctkWorkflowWidgetStep::name, description, statusText
into qt meta-object system to be accessible from QObject::property()
Add ctkWorkflowGroupBox::titleFormat, subTitleFormat and errorTextFormat

Give the option to configure the look of the ctkWorkflowGroupBox items.
Add ctkWorkflowStep* ctkWorkflow::step(QString)

Convenient method to search and return a step. This can be useful when a
step needs knowledge of another step.
Improve look&feel of ctkWorkflowGroupBox

* no internal margin around the step widget
* subtitle, pre, post and error texts are no longer user selectable