Use boost::filesystem paths

[rgrieder]

We've got lots of filenames and filepaths. And they might be Unix or Windows incompatible. Best solution would be to always use boost::filesystem::path to take care of all the platform specific stuff.

Unfortunately <boost/filesystem.hpp> has quite a large tail so including it in many source files takes a lot of time. I recommend not resolving this ticket unless we have support for precompiled header files.

[rgrieder]

For about two days the Core class has methods that return a reference to a boost::filesystem::path. That works without including the boost header (notice the forward declaration at the beginning of Core.h).
In order to store the paths I had to make a minor workaround using global variables that have internal linkage (that means they cannot be referenced from anywhere else than Core.cc). The usual way would have been to use a global variable that is declared in the Core class hence including boost/filesystem.hpp in Core.h

The following classes already make use of boost::filesystem: Core, CommandLine, ConfigFileManager, Language, ArgumentCompletionFunctions, LuaBind, GSGraphics and GUIManager.

[rgrieder]

Worth noting: fstream.open() works with Unix forward slashes on Windows. So you can easily use Core::getMediaPathString() (already includes a "/" at the end).

I therefore declare this ticket as more or less resolved since I don't see where else it would be really necessary to use boost::filesystem.