Fix saving and exporting in Pale Moon

[adeshkp]

Saving and exporting does not work in Pale Moon browser due to mxXmlRequest.simulate not working. Apply the same workaround as already done for Safari.

See: https://forum.palemoon.org/viewtopic.php?p=135614#p135614

[davidjgraph]

Thanks for the submission. The mxclient change would actually go in the core project, in Bitbucket, that's the built mxclient.js.

The reason for determining browsers by user agents is legacy. We've set a bad example here by doing it, so we shouldnt really complain when PRs do it. However, I'd love to see this done as a capability check, instead. Maybe it can't actually be done, but I'd prefer it to be tried before we resort to this method.

[alderg]

Thanks, this will be fixed in the next release.