Fix bug 9480: Labels for polygon centroids should always originate inside a polygon

This pull fixes the bug 9480 (http://hub.qgis.org/issues/9480).

Changes:
+ New 'pointOnSurface' method for QgsGeometry using 'GEOSPointOnSurface'.
+ New option for CentroidFillSymbolLayerV2 to force the location of centroid inside of polygons.
+ Force label position inside of polygons in PAL labeling library.