add alignment properties to QTextItem

I propose to include horizontal and vertical alignment properties in QTextItem, so that a QPainterEngine can be told to draw the given text with the given alignment relative to the given position.

The reason for my desire to have this feature is that I am using the QSvgGenerator to draw my QGraphicsScene into an svg file. In a second step I am using Inkscape to export this svg file into a latex compatible pdf-file, in which the text contents are replaced by the according LaTex Symbols. As during this replacement the width of the displayed text is very likely to change (especially for formulas) the text afterwards is no longer optimally displayed. As svg supports to align text item relative to a text-anchor, it would be very helpful to be able to use this feature when exporint from a QGraphicsScene.