Improve performance of the svgdigitizer for files with embedded figures

[DunklesArchipel]

The processing time of svg file with embedded figures is extremely high. Maybe there is a workaround to minimize the processing speed.

[DunklesArchipel]

The bottleneck is minidom.parse(self.filename)

[saraedum]

I think minidom is just not a very fast parser. Parsing the embedded image should be trivial as it only has to do base64 conversion I think. But understandably, performance was maybe not the main concern in that implementation.

http://elektito.com/2017/08/25/benchmarking-python-xml-parsers/

This is a bit old but lxml might be a good option that we could look into.