combobox—Always popup menu using current event

combobox—Always popup menu using current event

priv->trigger_event is never set, so it is always NULL. This means the
gtk_menu_popup*() methods use the current event. The only way to get any
other event to combobox_menu_popup() was from the button-press-event
handler I just removed, which would end up being the current one anyway.

So, bin priv->trigger_event & explicitly pass NULL to gtk_menu_popup*().