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 also seems like an API change to me, previously `'None'` was the canonical way to store 'don't draw anything' and now it is an empty string.