wizard: do not show 'show log' expander if log is empty

Log expander is invisible now by default and a callback to check if the
log has changed has been added.

Closes #153.

Signed-off-by: Petr Kubat <pkubat@redhat.com>
Merge pull request #208 from abrt/gh153

wizard: do not show 'show log' expander if log is empty