Add support to backup pages using API:Query instead of Special:Export
WikiTeam#280

into recent version of dumpgenerator.py

Adds additional parameter: --apiexport
which uses a query request instead of submit on api.php which works
without Special:Export which is disabled on some sites.

 -renamed --apiexport to '--apiquery' and gave it a more useful help text
 -renamed function 'getXMLPage_' to 'selectXMLQuerMode' to make it distinct enough from 'getXMLPage'
 -added in checks so that '--apiquery' can only be used together with '--curonly'
 -dialed back some timeouts from 120 to 60 which is still more than the previous 30

Improvements that could still be implemented:
 -Checking the availability of Special:Export before trying to download the list of titles and images wasting resources

'--images' and '--xmlrevisions' too, accidentally disabled that in
previous commit