XML import and output for Apostrophe

[boutell]

Apostrophe should define an XML format for importing and exporting sites, including history etc. Third party slot types could participate if they define methods for reading and writing representations of themselves in XML.

This would replace our current dodgy workarounds for Symfony's limited fixtures and even allow us an upgrade path if we switch to different database technology someday. But more importantly it would allow anyone to export their existing site to Apostrophe even if we have no native importer for their particular CMS. It would also allow a clean division of labor ("you build the exporter, we worry about making sure the importer works properly with properly formatted input").

[geoffd]

We have an importer now. An exporter still needs to be created. Importer can not handle blog posts yet.