UI Tasks view: fix per page settings

Configuration -> Tasks -> per page settings and page navigation
wouldn't work correctly:
* the view would always default to 50 items per page
* incorrect numbers shown next to the navigation icons

Fixes #818

https://bugzilla.redhat.com/show_bug.cgi?id=1171150
We also call change_tab() when navigating through the tasks list (those next, previous, first, last buttons).