Ai boost filesystem

AI's the boost filesystem branch contains 2 parts:
1) replace our filesystem.cpp with a boost based version (filesystem_boost.cpp) which supports utf8 on all systems. (that's in this pr)
2) make SDL use our filesystem functions (that's not in this pr)
in order to gain utf8 support on window we need both.


the filesystem_boost.cpp file replaces the filesystem.cpp but just like in ai's original branch this still leaves the original filesystem.cpp file so one can choose whether to compile filesystem.cpp or filesystem_boost.cpp. Thats also why i have less concerns abouth merging this untested branch.