move shift_months to liboffsets, unify implementations of Q/M/Y offsets,
This moves `tslib.shift_months` to liboffsets.

After several algebra-simplifing PRs, here's the payoff.  Nearly all of the logic in M/Q/Y offset subclasses is now identical, so can be moved into the parent classes.

`BusinessMonthEnd` gains an `onOffset` method (instead of inheriting from `DateOffset`)

The Year and Day algebra is simple enough that the diff should be obvious.  The `QuarterFoo.apply` methods are require a few steps of simplification before they become identical.  I decided just to push the whole thing rather than drag it out.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=h1) Report
> Merging [#18375](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a172ff9dd32d7f6ba077c82b6cf6a31dcc7f2b45?src=pr&el=desc) will **decrease** coverage by `0.06%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18375/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18375      +/-   ##
==========================================
- Coverage   91.38%   91.32%   -0.07%     
==========================================
  Files         164      164              
  Lines       49797    49703      -94     
==========================================
- Hits        45508    45391     -117     
- Misses       4289     4312      +23
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.12% <100%> (-0.05%)` | :arrow_down: |
| #single | `39.61% <35.71%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.7% <100%> (-0.23%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.44% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `92.52% <0%> (-0.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18375?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

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