Use Kano library's scrolled window for listing users

KanoComputing/peldins#1523
Use scrolled window provided by Kano's Gtk library to standardise
styles. Also increase the height of the user list frame so that there is
room for 2 users before having to scroll.
Merge pull request #3 from KanoComputing/scrollbars

Use Kano library's scrolled window for listing users