Drop events updating/stacking support

Get back to client-side support

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Remove the action message type

Also add the missing highlight command for the exec plugin

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
plugin: Add a macro helper for callbacks calls

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Remove "special" events

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
plugin/utils: Fix function usage

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Use PURPLE_EVENTS_PLUGIN_ID in plugin descriptions structures

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

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