Fix build on OS X with Clang.

Update the virtual file systems to handle right locations starting with "//"
(by ignoring the //), and make all requests using locations in this form, to
avoid NSURL percent encoding the string (which was ruining the hash tag we pass
from the StateGraphViewer).

Display the debug window (if enabled) before doing any other processing.

Print some debug information when the StateGraphViewer's wxWebView raises
events NAVIGATED, LOADED, or ERROR.