Refactor|Plugins: Renamed WadMapConverter as IdTech1Converter

Importing (and perhaps converting) of id Tech 1 format resources is
in the process of being moved out of the engine. It makes sense to
collect all such resource importers into a single plugin as they'll
likely share many components that are only needed here.

This change in scope therefore warrants a plugin rename and version
number reset.

Windows: The installer script was updated to automatically remove the
old version of this plugin if found during install/upgrade.