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.
@mdboom @efiring This breaks the doc-string scraping code for the tables in the docs.  Should we revert the changes to put `ACCEPTS : ...` back in or re-work the doc-scraping?