examples: Make hello-c-gnome3 translator friendly

examples: Make hello-c-gnome3 translator friendly

Suggested by Benno Schulenberg in:
<https://lists.gnu.org/archive/html/bug-gettext/2015-01/msg00003.html>.

* hello-c-gnome3/hello.gschema.xml: Use "use-markup" property
instead of "sensitive", to clarify the meaning of the extracted
strings.
* hello-c-gnome3/hello.c (HelloApplicationWindow): Rename the
"label2" member to "label".
(update_content): New function.
(clicked_callback): Rename from quit_callback and call
update_content instead of quitting.  All callers changed.
* hello-c-gnome3/hello.ui: Abolish the first label and put a
translatable text inside the button.