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 definitly should not go there as I don't think this is going to be back-ported to the 1.4 series.