Use the UTF-8 version of Expat

Expat provides better support for its UTF-8 API than its "unicode" (```wchar_t``` based) API. This allows us to use expat 2.1's CMake-based build system, which doesn't even support the ```wchar_t``` API currently. It also avoids the manual override we were previously using to tell it we were using the ```wchar_t``` version instead of the default.  **Please note that this does not require a change of the localization XML data itself**, as expat will still parse the data in whatever format it reads -- this only changes what format the parsed data is presented to client-side code (and our internal storage is already ```plString```, which uses UTF-8 internally)

***VERY IMPORTANT NOTE:***  This will require a new build of your expat library!  This will NOT BE OBVIOUS, since it will still compile and link, but will give you junk localization data :(