Topicmaps API: post_refresh_topicmap_menu (#646).

New Webclient event:
{{{
"post_refresh_topicmap_menu"
}}}
It is fired each time the topicmap menu is build. One argument is passed to the event handler: the topicmap menu.

This event allows 3rd party plugins to attach it own commands to the topicmap menu. Example:
{{{
dm4c.add_listener("post_refresh_topicmap_menu", function(topicmap_menu) {
    topicmap_menu.add_separator()
    topicmap_menu.add_item(...)
})
}}}

See #646.