StButton: fix handling of Space/Enter -> click

StButton: fix handling of Space/Enter -> click

StButton was mistakenly considering any Space/Enter KEY_RELEASE to be
a click, when in fact it should only count as a click if it also got
the corresponding KEY_PRESS as well. This meant that when typing in a
chat notification, any Space/Enter keypress would dismiss the
notification, since the StEntry would take the PRESS event but ignore
the RELEASE, allowing it to propagate to the notification itself,
which would treat it as a click.

https://bugzilla.gnome.org/show_bug.cgi?id=645243