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

... for collection, lines and patches.

(code & tests)

see. #2136

This is a minimal patch. It seems to me that the linestyle handling could get some more cleanup.
e.g. in lines.py the drawing is handed over to some `_draw_xxx` methods, those are almost only stubs changing the name of the linestyle
is it okay to have a linebreak here like
```
Parameters
----------
ls : { '-',  '--', '-.', ':' , 'solid', 'dashed', 'dashdot', 'dotted',
       (offset, on-off-dash-seq)}
    The line style.  [[EXPLAIN on-off-dash-seq]]
```