workflow config: use scrollbars instead of enormous window size

Scrollbars has been chosen instead of displaying an event configuration
in a separate tab because any event option can expand dialog size to
enormous size. Therefore separate tabs requires scrollbars too.
Even more the tabs are less clear than displaying all options on a
single window.

Closes #957997

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Martin Milata <mmilata@redhat.com>