[Stop using ATL] Replace ATL::CString with std::wstring

[eric@adblockplus.org]

Background

We are using ATL::CString extensively. It offers no substantive advantages over std::wstring. Its use induces a lot of clutter in the form of not-very-necessary type conversions.

What to change

Replace all uses of CString with std::wstring. While not particularly technically difficult, there are a few complications.

• There are a handful of CString member functions that don't have equivalents in wstring. These require replacement.
• It's not possible to perform the conversion all at once. During the interim, we'll using explicit conversion functions that mark the boundary of progress for this change.
• The logging functions for debug use CString at present but will require both wstring and string signatures after conversion. Most will use the wstring version, but std::exception::what() uses string. It's expedient to make a small exception here.

[eric@adblockplus.org]

As of the commit of ​https://codereview.adblockplus.org/29347788/, the only remaining uses of CString are in the threading code for deferred log writes.