Journal multiselection try2

This is a first try to push to the main repository a feature developed two years ago by Martin Abente, and improved by AC developers to be included in Dextrose.
This implementation use part of that, but improve in several points.
* Performance improved by adding a method in sugar-datastore, to get the uids in a query without pagination.
* The batch operations can be interrupted.
* Many changes in the code to make it more modular.

This feature need the following related pull requests:
in sugar-datastore:
https://github.com/sugarlabs/sugar-datastore/pull/1

in sugar-artwork:
https://github.com/sugarlabs/sugar-artwork/pull/7