webadmin: image cell - show tooltip only when needed

StyledImageResourceCell ->
title panel should be visible only when a title
is available (i.e. != empty string).

Example screenshot: http://i.imgur.com/oem3uNH.png
Change-Id: If3ea9edf883dfffe59996ef31efe96a6d60f6ccf
Signed-off-by: Daniel Erez <derez@redhat.com>