Move some includes from the header to the source.
Add a new wxWidgets virtual file system handler which allows "files" to be
registered as callback functions (using std::function), which are called each
time the file is requested to find the value of the file. This implementation
is focused on allowing our WebView's Javascript to request information from the
C++ environment, so the function results are packaged into JSON format. At the
moment the callback arguments and results can only be primitive numeric types.