submodule inheritance

[casperdcl]

missing from #800, fixes #790

• update gui inheritance
• possibly Modularize tqdm.pandas -> tqdm_pandas #271 tqdm.pandas -> tqdm.pandas.register(tclass=tqdm.std.tqdm)? decorator?
• delayed imports (should not import optional dependencies until actually necessary #176, 2nd issue in AttributeError: type object 'tqdm' has no attribute '_instances' #188)
• future submods as per Polynomial regression to predict time (tqdm_regress, tsrange) #248, Add tqdm_bare and tbrange as minimal example for ports developers #258, Dask support #278, Bars #227, Adjust EMA smoothing coefficient #609, Implementation of pessimistic progress rate #158, Web-friendly tqdm #94, Create progress bar wrapper to use with dask #279, Add support for data streams (with known total size) #84, TqdmUpTo, Pythonic submodules architecture #198 (comment) (simple), Pythonic submodules architecture #198 (comment) (compat, perf, dload)
• consider major release v5 #561