Added include path resolution to FilesystemContext constructor
* extracted system id handling into separate "resolve" method overload of "IncludeEntityResolver" class
* paths to be resolved against the include path vector are enclosed by square brackets
** FilesystemContext for external functions was instantiated using the unmodified systemId provided by xalan::Locator
** this commit adds include path resolution to that step
*** exceptions were thrown as the program tried to instantiate a FilesystemContext instance without resolving the square bracket syntax
* changed FunctionTransform "target" attribute content to the target path filename instead of the full path