New optional method DiaObject::transform() to apply an affine transformation
to the object. Immediately visible effect - without SVG - is rotation support
for 'Standard - Box' and 'Standard - Ellipse'.
'Standard - Text' and 'Standard - Image' are not fully supported yet, because
they require significant work on the renderer level.
Standard - Arc, Outline and ZigZagLine are not included because they wont be
used from SVG and the direct support is considered pointless.

Currently the intended main use-case is improved transformation from SVG
especially on the level of single elements. The included test file
transform-variations.svg shows the issues solved and open.