Allow to load an XML-stored mxGraphModel produced by mxGraph #293

mxGraph provides an exchange format that let import/export its model in XML form.
Allowing loading/importing of the old mxGraph format to feed the maxGraph model would make it easier for former mxGraph users who rely on this functionality to migrate to maxGraph.

DISCLAIMER: this issue is only about allowing to load mxGraphModel.
Other XML encoded elements coming from mxGraph may be loaded, but this is out of the scope of this issue.

Relates to #221
See also #169, #178, #197

Notes

A "stashed" Codecs example includes a mxGraph model stored in the XML form. This example should be migrated to Storybook to demonstrate the new feature.
#221 also provides an XML string of the mxGraph model.
#102 (comment) gives an example of an mxCell with a style attribute.