Setting the correct work directory in FunctionExternalCommand
* changed the XObjectValue constructor to accept a pointer to FilesystemContext instead of instantiating the context by itself
* the FilesystemContext is now instantiated in the "callConstructDocument" member method of "FunctionBase"
** a const reference to FilesystemContext is passed to all "constructDocument" member method implementations
* the FilesystemContext is currently only used by FunctionExternal command to set the correct work directory in "boost::process::context"
** this is required so calling external commands from inside a stylesheet works as expected
*** i.e. from inside the directory the stylesheet is located in
* modified all remaining external function implementations accordingly