First stab at smoothing associationwidget endpoint update as announced

  in https://bugs.kde.org/show_bug.cgi?id=447866#c5

umbrello/umlwidgets/associationwidget.{h,cpp}
- At function calculateEndingPoints add argument UMLWidget *pWidget
  indicating the selected widget of move or resize.  If passed in as
  null then updateAssociations() will be called twice, once for the
  current associationwidget's role A widget and again for its role B
  widget.
- Add function findIntercept(const QRectF& rect, const QPointF& point,
                                                       QPointF& result)
  returning bool true if the line from the center of `rect' to `point'
  intersects with one of rect's sides, and returning the intersection
  point in `result'.
- Remove functions updateRegionLineCount and doUpdates.
- Declare function updateAssociations `static' and change arguments to
    UMLWidget *pWidget, AssociationWidgetList list.
  For details see documentation at the implementation.

CCBUG: 447866