Use GitHub XSDs instead of internal VS repo ones

[rainersigwald]

Long-term fix for #1488, which was mitigated by a manual sync of the XSDs to the VS repo for v15 but should be fixed permanently by deleting the XSDs from VS and deploying them from GitHub to the right place in our VSIX.

[rainersigwald]

Note that this should include LOCALIZATION of the XSDs, which is currently done on the copy in the VS repo.