Validate SSL against root certificates

Validate SSL against root certificates

If one of two known root certificate files is available, we
enable certificate validation, otherwise a warning is emitted.
Not using validation means that we trust any certificate. We
should consider doing the opposite.

The new property MidoriView:security is set during load
and indicates the security status. midori_view_get_security can
be used to determine the status later on.

midori_location_action_set_security_hint is implemented to
present security status visually. For now, this only changes
the icon, we need to find a more visible solution.

MidoriBrowser updates the security hint for the current view in
_midori_browser_update_interface.