BaseImporter::SearchFileHeaderForToken() expected a pointer to a non-const token list. This was probably an oversight, as nobody would realistically expect the function to change the list. Furthermore, it prevented token lists from being compiled to read-only memory, in some cases even causing the compiler to generate thread-safe initialization.
The list is now const and all callers declare their token lists static const, thus compiling them to read-only memory.
The DXF importer defined a global std::string constant, only to convert it back to a C string on use.
This commit defines the constant as a C string right away, thus saving 340 B of code and data.
