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.
Well dashes always contains a list of dash patterns, one for each part of the collection. A dash patterns is a 2-tuple, so if you pass in a 2-tuple it is converted into a list of one 2-tuple.