Add a new method hildon_banner_show_information_override_dnd

Add a new method hildon_banner_show_information_override_dnd

* hildon/hildon-banner-private.h: new require_override_dnd and
overrides_dnd variables
* hildon/hildon-banner.h:
(hildon_banner_show_information_override_dnd): new method, that shows
the information but sets the override flag, in order to show important
messages
* hildon/hildon-banner.c
(hildon_banner_show_information_override_dnd): new method
(hildon_banner_set_override_flag): new utility function
(hildon_banner_real_show_information): new utility function, in order to
generalize the current hildon_banner_show_information_override_dnd and
hildon_banner_show_information_override_dnd
* examples/Makefile.am
* examples/hildon-banner-dnd-example.c: New example that show how to
use the new hildon_banner_show_information_override_dnd

Fixes: NB#114874 (There is no possibility to show hildon banners with
DND flag set)