Update for libeventc upstream changes

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Event update/stacking support removed from purple-events

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Move from purple-events callbacks to libpurple signals

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>