gnulib-tool.py: Add a new GLFileTable class. * pygnulib/GLFileTable.py: New file. Define the GLFileTable class with five attributes which can be individually typed. * pygnulib/GLTestDir.py (GLTestDir.execute): Use the GLFileTable class. * pygnulib/GLImport.py (GLImport.gnulib_comp, GLImport.prepare) (GLImport.execute): Likewise. Update type hints and doc strings.