Implement Interpreter's InclusionDirective() callback

InclusionDirective() callback tells us that an inclusion directive has been processed, allowing us to try to autoload classes using their header file name.
For example try to autoload TGClient (libGui) when seeing #include "TGClient.h"
This should prevent potential crash when executing macros using globals (e.g. gClient), with the following error:
ExecutionContext: use of undefined symbol 'gClient'!