fixed at 90 dpi - not compatible with inskcape 0.92

[jnweiger]

With upcoming inkscape 0.92 the internal dpi setting changes from 90 to 96.
This extension uses a hardcoded 90.
See also t-oster/VisiCut#347

[jnweiger]

It does not work reliably:

1. When the document was saved or loaded before an extension is called, then we have elements inkscape:version="0.92.1 unknown" sodipodi:docname="box.svg"
   in the document. If it was never saved, both are missing, even with inkscape-0.92

2. if inkscape:document-units="mm" the px dpi should not matter. Currently they do.

[jnweiger]

Workaround works when we have a viewbox. Still possibly an inkscape bug? There should not be a difference before and after saving!