Topicmaps: prepare for renderer customizers (#518).

The initial topicmap is rendered at "init_3" time.
This allows 3rd party plugins to register their topicmap renderer customizers at "init_2" time. That is before the initial topicmap is rendered.

Furthermore: the Topicmaps plugin provides a public `get_topicmap_renderer()` method:
{{{
function get_topicmap_renderer(renderer_uri)
}}}

It allows 3rd party plugins to access the installed topicmap renderers in order to add customizers to them (yet to come). 3rd party plugins access a renderer like this:
{{{
dm4c.get_plugin("de.deepamehta.topicmaps").get_topicmap_renderer("dm4.webclient.default_topicmap_renderer")
}}}

Close #518.