CLN: move plotting funcs to pd.plotting
Rebased in #16005

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- [x] closes #12548
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

cleanup `tools/plotting` as `plotting` subpackage. Because the differences are being huge, would like to split 3 part if base direction is OK:

~~1. Move plotting related tests under `pandas/tests/plotting` splitting to `Series`, `DataFrame`, etc. (0.18.2)~~ (#13621)
2. Create `pd.plotting` and move related files (0.20.0) 
3. Split / refactor `pd.plotting` to use small utility functions / property control classes. Add tests. (0.20.0)
