Merge pull request #246 from dpogue/plfile

Move file stuff back to plFile for gcc/clang.
Add UnicodeBuffer as first class data type and allow plStrings to be constructed with it
Clean up some useless junk from plString
Use plString in plConfigInfo and friends
Remove xtl::istring and xtl::iwstring
Remove an unnecessary extra heap allocation in plString::Format
Replace xtl::delete_ptr functor with lambdas
Remove xtl::copy_if because:
- It is included in C++11
- Nobody was using it anyway >_<
Add identification for some more modern Windows versions
Remove unused headers from hsStlUtils.h (and push them downstream where needed)

Also fix a couple of plString and header issues identified by Paradox on GCC/Clang
Deprecate and remove xtl::format in favor of plString::Format
Fully remove hsStlUtils, as it is now completely unused
Deprecate and remove hsStlSortUtils