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
This is an api break on a public part of the API.  It seems we don't use it anythere else in the code base (and hopefully no one else does either!) this needs to be documented in api_changes.rst.

While touching this, might as will make it private (add an `_` to the name).