Uses `const std::string&`

... instead of const char*

* Because it is more robust
* and it is faster... yes faster.

It is faster because internally some of our classes use std::string
(like FileUtils and Dictionary) so converting back-and-forth from const
char * to std::string is very expensive.

It is faster to have everthing in std::string as soon as possible to
prevent further conversions.

There are still some parts of the code that require conversion, but
most of the code was converted.
Merge pull request #4102 from ricardoquesada/uses_string

Uses `const std::string&`