Add a little function used to convert if needed the size of a font set
in dpi to pixel. Because the conversion from pdi to pixel can't
be exactly identical, the text size in the diagram can be a little
different, in the other side the switch between screens with different
dpi is no more a problem.
Add a combo box in the tool bar of diagram editor
to quickly change the size of the graphics handler item.
The sarto commit :D
NOTE
only available for diagram editor, element editor will
come later.
