Allow both linestyle definition "accents" and dash-patterns as linestyle

This is a first PR in an attempt to split #3265 in smaller pieces.
This part should cover the original intent to fix #2136.

The reverse mapper is necessary here to convert full line style names in short ones for lines.py.
```python
In [2]: ln, = plt.plot(range(5))

In [8]: ln.set_linestyle('aardvark')

In [9]: plt.draw()
```

does not raise an exception, but I think it should.

I have a feeling I suggested you make this change and have now changed my mind about it, sorry.