rewrite backup procedure, use XmlSerializer

Use the `org.xmlpull.v1.XmlSerializer` to rewrite backup to GncXML procedure. Now it exports much faster, and won't get out of memory error #186 on my big book.

Tested on single currency book without recurrence transactions, deleted all accounts and restored backup, results seem to be OK.

This export approach only works with the new bulk import #188 , as the accounts are not ordered when exporting. The original import procedure requires the accounts to be ordered to get correct full name.

Export to zip is not added yet, but should be easy to add.

Export and import of "exported status" #192 is not included.