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.
It should be more-or-less by date.  Can you actually throw it into this folder (https://github.com/matplotlib/matplotlib/tree/master/doc/api/api_changes)?   This is an attempt to alleviate the merge conflicts in the docs.