link-button: Add ::activate-link signal

link-button: Add ::activate-link signal

We cannot use the GtkButton::clicked signal to override the default
behavior of GtkLinkButton (i.e. call gtk_show_uri()), because
GtkButton::clicked is registered as a RUN_FIRST signal, which obviously
prevents any other signal handler connected to it to stop the
propagation before the class handler has a chance to run.

For this reason we can add a GtkLinkButton::activate-link signal, which
will be emitted by the default GtkButton::clicked signal handler; the
::activate-link signal has a boolean return value, which allows simpler
code for stopping the propagation to the next signal handler.

https://bugzilla.gnome.org/show_bug.cgi?id=632150