Added support for resolving non-existing paths
* previous logic for resolving boost::filesystem::path parameters in the XObjectValue class actively tried to resolve existing files
** this contradicts the planned introduction of e.g. a external "write-file" function
* callers of external functions with path arguments now have to enclose them in square brackets if include path resolution is required
** analog to the usage of the "xsl:import" tag
* moved "getPathFromSystemId" from compilation local method into static method of IncludeEntityResolver
* changed test cases accordingly