Blender Git Loki

Replace char * with string in texture map types.

Review update rB827869a45bccbf1e016580b#271046 (by Jacques Lucke)

As mentioned in his comments, `Map` will do a pointer comparison for
`char *` keys. So string would be better. Even though the `lookup` has
been removed, `std::string` is still okay.

Also instead of iterating over `Map.keys()`, and then looking up value,
now it's iterating over a key-value pair: `Map<T, T>::item` of strings.

Minor comment changes.