/**
* CompletePathWithAppPath
* All the components that are specified in the given descriptor (drive letter,
* path and file name, including extension) are put into the result;
* any missing components (path and drive letter) are taken from the app's path.
*
* Can be used e.g. to load a bitmap file when an application don't know where
* it has been installed.
*
* Example1:
* TFilename fname = _L("\testdir\pics.mbm"); // Use _LIT instead
* CompletePathWithAppPath( fname );
* Result:
* fname == "c:\testdir\pics.mbm" if application was installed to c:
*
* Example2:
* TFilename fname = _L("pics.mbm"); // Use _LIT instead
* CompletePathWithAppPath( fname );
* Result:
* fname == "c:\system\apps\myapp\pics.mbm" if application was
* installed to c:\system\apps\myapp
*
* @param aFileName FileName which will be completed with application's path
* @return Error code if an error occured. In case of an error aFileName will
* not be changed
*/