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()

Co-authored by: Johan Andruejol <johan.andruejol@kitware.com>
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
Merge branch 'ctkworkflow-tweaks'

* ctkworkflow-tweaks:
  Improve look&feel of ctkWorkflowGroupBox
  Add ctkWorkflowStep* ctkWorkflow::step(QString)
  Add ctkWorkflowGroupBox::titleFormat, subTitleFormat and errorTextFormat
  Add ctkWorkflowButtonBoxWidget::[back|next|goTo]ButtonFormat
  Fix workflow next button arrow and text
  Add ctkPushButton, an advanced QPushButton
  Fix ctkWorkflowButtonBoxWidget button size policy
  Add ctkWorkflowButtonBoxWidget::HideGoToButtons
  Cleanup ctkWorkflowButtonBoxWidget