Mostly purge wxWidgets from non-UI stuff

Use boost::filesystem::path for all paths, and std::string for all other
strings, converting to/from wxString as close to the actual uses of wx
as possible.

Where possible, replace the uses of non-UI wxWidgets functionality with
the additions to the standard library in C++11, or the equivalents in
boost.

Move the path token management logic to libaegisub (and rewrite it in
the process).

Add a basic thread pool based on asio and std::thread to libaegisub.

This touches nearly every file in the project and a nontrivial amount of
code had to be rewritten entirely, so there's probably a lot of broken
stuff.