#250 doctools - Use pluginmgr to handle backends

[Andreas Kupries]

The backends are currently handled through explicitly created safe interpreters, which are not very good are loading packages or other code.

Rework the doctools internals to use Tcllib's pluginmgr package to handle checker layer and backends.

Backends and common supporting code can become proper packages (Tcl Modules even), much easier to manage.

Note: Message catalogs can get a package interface too. Load package, adds path to look at for the catalogs. alternate: Load package, gets the catalog.