Expose and use Engine's page size variable

Engine has a pageSize variable which has
been mostly unused, but which comes in very
handy when getting large sets of data. This
patch exposes that variable through a
setter, and makes use of it internally
whenever a request is made (so the page
size is actually honoured the way it seems
to have been intended)

Differential Revision: https://phabricator.kde.org/D6190