Switch to data.xml for SVG transformation to fix label offset in plots

[glts]

This change switches to data.xml for SVG transformation.

data.xml preserves the whitespace from the source document, so this should fix #17.

• I am depending on data.xml 0.2.0-alpha5 because only the 0.2.0 series includes XML namespace support, which is needed for SVG. As far as I know, the alpha is quite stable.
• I made one change to preserve metadata during the ID mangling. data.xml needs this metadata to generate the right XML namespace prefixes during emitting.

The worksheets I tried (and examined) with different plots etc. all look good, so this seems to completely resolve the issue!

[glts]

Hello @sbelak,

I think this change (or a similar one) would be quite valuable, because it fixes a visual bug that’s noticeable in all Huri plots today. The plots are a bit awkward to use in presentations for example, because all the labels are slightly off.

I would be very happy if you could consider including it.

Thank you, David