Fixed crashes in Observer mode related to infinite bounding rects

Fixed crashes in Observer mode related to infinite bounding rects

Bug fixed by avoiding uniting or subtracting QPolygonF with potentially
infinite coordinates.

The LiveSelectionIndicator now uses a QGraphicsRectItem rather than a
QGraphicsPolygonItem and displays only the boundaries of selected
objects, not including their children.

The SubcomponentMaskLayerItem now works with rectangles and uses a
QRegion to determine the area around the current context, converting
this to a polygon only as a last step.

Reviewed-by: Kai Koehne
Task-number: QTCREATORBUG-4559
Change-Id: I266f5387fa67017fc50215282a95b4ee6498be6d
(cherry picked from commit d03065da2999b8539d8c5160b58d56dd94373d6f)