Remove use of operator[] on a std::map, so no spurious elements get inserted. (fixes #464)

All uses of operator[] in guiConfigureWorld.cpp have been replaced
with .find() operations.

In some places, operator[] had been called with mod names of "Add-Ons"
or "N/A", which would insert a default-constructed ModSpec into the
list of add-on mods. In other places, the use of operator[] was safe
now, but it probably is safer for future changes to use .find()
everywhere.